Warfarin

Mechanism

Warfarin acts via two stereoisomers: S-warfarin (5-fold more active) and R-warfarin. Amiodarone strongly inhibits CYP2C9 (the S-warfarin pathway) and, to a lesser extent, CYP3A4 (the R-warfarin pathway). The active form accumulates; the international normalised ratio (INR, a marker of blood clotting) rises 2- to 4-fold. The effect develops slowly over 1–7 weeks because amiodarone has a very long half-life.

Symptoms

Gum bleeding, epistaxis, bruising without trauma, blood in urine or stool, menorrhagia. Severe cases include gastrointestinal and intracranial haemorrhage with fatal outcomes – reported at INR above 10.

Management

The combination is acceptable when amiodarone cannot be replaced by another antiarrhythmic. Reduce warfarin dose empirically by 30–50% at the time amiodarone is added. Check INR every 3–5 days for the first month, then every 2 weeks. The interaction persists for several weeks after amiodarone is stopped – return warfarin to baseline dose gradually.

Mechanism

Warfarin blocks vitamin K-dependent clotting factor synthesis (II, VII, IX, X); apixaban directly inhibits factor Xa. The two different mechanisms add up.

Symptoms

Gum bleeding, epistaxis, bruising without trauma, blood in urine or stool, menorrhagia. Severe cases include gastrointestinal or intracranial haemorrhage, gross haematuria with renal colic, spontaneous retroperitoneal haematomas. Symptoms may appear within days of co-prescription.

Management

The combination is not prescribed. Switching from warfarin to apixaban: stop warfarin, wait for INR below 2.0, then start apixaban. Switching from apixaban to warfarin: overlap with INR monitoring, stop apixaban once INR exceeds 2.0.

Mechanism

Capecitabine raises INR 4-fold or more via CYP2C9 inhibition – EMA Xeloda Section 4.4 boxed warning.

Symptoms

Severe bleeding within first weeks of course.

Management

Check INR every 3-4 days. Switch to LMWH preferred.

Mechanism

Warfarin blocks vitamin K-dependent clotting factor synthesis; dabigatran is a direct thrombin inhibitor. The two mechanisms are additive with bleeding risk.

Symptoms

Gum bleeding, epistaxis, bruising without trauma, blood in urine or stool, menorrhagia. Severe cases include gastrointestinal or intracranial haemorrhage, gross haematuria with renal colic, spontaneous retroperitoneal haematomas. Symptoms may appear within days of co-prescription.

Management

The combination is not prescribed. Switching from warfarin to dabigatran: stop warfarin, wait for INR below 2.0, then start dabigatran. Reverse switch: start warfarin 3 days before stopping dabigatran if creatinine clearance is over 50 mL/min, 2 days if 30–50 mL/min, 1 day if below 30 mL/min.

Mechanism

5-FU inhibits CYP2C9 (S-warfarin metabolism) and impairs vitamin K-dependent clotting factor synthesis through direct myelotoxicity. INR rises 2-4 fold within 5-10 days. Fatal bleeding cases reported.

Symptoms

Gum/nose bleeding, haematuria, bloody stool within 5-10 days. INR above 5.0.

Management

If possible – switch warfarin to LMWH during chemotherapy. If warfarin needed – reduce by 30-50% with weekly INR.

Mechanism

Ketorolac inhibits platelet aggregation, prolongs bleeding time and damages the gastric mucosa; according to its SmPC it causes serious gastrointestinal toxicity more often than equivalent doses of other NSAIDs. NSAIDs enhance the effect of coumarin anticoagulants such as warfarin on bleeding time, and patients taking warfarin with an NSAID have a higher risk of serious gastrointestinal bleeding than with either drug alone.

Symptoms

Gum bleeding, epistaxis, bruising without trauma, blood in urine or stool, menorrhagia. Severe cases include gastrointestinal or intracranial haemorrhage. Risk rises in patients over 65 and with prior peptic ulcer disease.

Management

The combination is contraindicated: Spanish ketorolac SmPCs (10 mg tablets, 30 mg/ml injection) contraindicate ketorolac in patients on coumarin anticoagulants, and warfarin is one of them (ketorolac SmPCs). The originator SmPC (Toradol) lists anticoagulants among combinations that are not recommended and asks for maximum caution with warfarin (Toradol SmPC). The FDA allows such therapy only with extreme caution, after weighing benefits against risks and with close monitoring (US ketorolac label). If the patient has already received ketorolac, stop it at once if gastrointestinal bleeding is suspected (ketorolac SmPCs); whenever a drug is started or stopped in a patient on warfarin, monitor the INR closely (warfarin SmPC).

Mechanism

Miconazole inhibits the liver enzymes CYP2C9 and CYP3A4 (miconazole labels), and these are the enzymes that break down warfarin. CYP2C9 breaks down the more potent S-enantiomer, and CYP1A2 and CYP3A4 the R-enantiomer (warfarin label, US Food and Drug Administration, FDA). According to the FDA warfarin label, inhibitors of these enzymes increase warfarin exposure and the international normalised ratio (INR). The Spanish warfarin label also lists miconazole among drugs that displace warfarin from plasma protein binding and potentiate its effect. The size of the effect depends on how much miconazole reaches the blood. A 60 mg dose of oral gel gives peak plasma levels of 31–49 ng/mL, and a 1200 mg vaginal ovule about 10–12 ng/mL. Less than 1% of a cream dose is absorbed (miconazole labels, Spanish Agency of Medicines and Medical Devices, AEMPS). The half-life of miconazole after oral gel is 20–25 hours (miconazole oral gel label, AEMPS).

Symptoms

The risk is bleeding: blood in the urine or stool, black tarry stools, heavy menstrual bleeding, petechiae, excessive bruising, prolonged bleeding from cuts, bleeding gums and nosebleeds (warfarin label, US Food and Drug Administration, FDA). Bleeding can also show as pain in the head, chest, abdomen or muscles, dizziness, shortness of breath, difficulty breathing or swallowing, unexplained swelling, weakness, low blood pressure or shock (warfarin label, Spanish Agency of Medicines and Medical Devices, AEMPS). Bleeding events, some fatal, have been reported with miconazole oral gel and warfarin (miconazole oral gel label, AEMPS). Bleeding and bruising have been reported after topical, vaginal and oral miconazole (miconazole buccal tablet label, FDA).

Management

Warfarin is contraindicated with systemic miconazole and with miconazole oral gel (warfarin label, Spanish Agency of Medicines and Medical Devices, AEMPS). The miconazole oral gel label also contraindicates the gel with warfarin (AEMPS). Miconazole skin creams, spray and vaginal ovules are not contraindicated. Use them with warfarin cautiously, monitor the anticoagulant effect closely and titrate the warfarin dose (miconazole cream and vaginal ovule labels, AEMPS). Explain the signs of bleeding to the patient and advise them to stop miconazole at once and contact a doctor if these appear (same labels). Check the international normalised ratio (INR) more often when any antifungal is started or stopped (warfarin label, US Food and Drug Administration, FDA). The Spanish warfarin label requires strict coagulation monitoring whenever any drug is added or withdrawn (warfarin label, AEMPS). If miconazole oral gel and warfarin have been combined, monitor the prothrombin time or other suitable coagulation tests closely and adjust the warfarin dose (miconazole oral gel label, AEMPS). In warfarin overdose with an INR below 6 and no bleeding, stop warfarin temporarily and restart at a lower dose guided by the INR. With an INR above 6 and no severe bleeding, give 0.5 mg of vitamin K1 by continuous infusion over 20–30 minutes, or 1 mg if the INR reaches 10 (warfarin label, AEMPS). In severe bleeding, give 10–20 mg of vitamin K1 by slow infusion over 1 hour together with fresh frozen plasma, whole blood or a factor IX complex (warfarin label, AEMPS).

Mechanism

Warfarin blocks vitamin K-dependent clotting factor synthesis; rivaroxaban directly inhibits factor Xa. Effects are additive with risk of major bleeding.

Symptoms

Gum bleeding, epistaxis, bruising without trauma, blood in urine or stool, menorrhagia. Severe cases include gastrointestinal or intracranial haemorrhage, gross haematuria with renal colic, spontaneous retroperitoneal haematomas. Symptoms may appear within days of co-prescription.

Management

The combination is not prescribed. Switching from warfarin to rivaroxaban: stop warfarin, wait for INR below 3.0, then start rivaroxaban. Reverse switch: overlap with INR monitoring, stop rivaroxaban once INR exceeds 2.0.

Mechanism

Aspirin irreversibly blocks platelet cyclooxygenase and reduces aggregation. Warfarin blocks vitamin K-dependent clotting factor synthesis. Two different antithrombotic mechanisms add up. Aspirin also injures the gastric mucosa, raising bleeding risk independently of coagulation parameters.

Symptoms

Gum bleeding, epistaxis, bruising without trauma, blood in urine or stool, menorrhagia. Severe cases include gastrointestinal or intracranial haemorrhage. Risk rises in patients over 65 and with prior peptic ulcer disease.

Management

High doses of aspirin and other salicylates are contraindicated with warfarin (warfarin SmPC). Analgesic doses of aspirin are not recommended with warfarin and are used only when there is no alternative (aspirin 500 mg SmPC). Aspirin is added to warfarin deliberately, for specific indications: the US warfarin label (FDA) allows it with prosthetic heart valves depending on valve type and position, and for high-risk patients after myocardial infarction lists warfarin plus aspirin up to 100 mg a day. With this combination, monitor the INR carefully (aspirin 100 mg SmPC) and watch for signs of bleeding (US warfarin label). Tell the patient not to start or stop aspirin or other salicylates, including over-the-counter products, without the doctor's advice (US warfarin label).

Mechanism

Carbamazepine strongly induces CYP2C9 and CYP3A4. Warfarin plasma levels fall by 50–70% over 2–4 weeks; anticoagulant effect weakens.

Symptoms

INR drops below target. Thromboembolic risk rises: deep vein thrombosis, pulmonary embolism, ischaemic stroke in atrial fibrillation.

Management

After starting carbamazepine, check INR every 1–2 weeks for the first month and titrate warfarin (usually 1.5- to 2-fold dose increase). On carbamazepine withdrawal, induction persists for 2–4 weeks – return warfarin gradually. Alternative anticonvulsants without induction: levetiracetam, lamotrigine.

Mechanism

Celecoxib and warfarin together have a synergistic effect on bleeding (US celecoxib label, FDA). With the combination, increases in prothrombin time (international normalised ratio, INR) and serious, sometimes fatal, bleeding have been reported, mostly in older patients (celecoxib SmPC). Like other non-steroidal anti-inflammatory drugs (NSAIDs), celecoxib can itself cause ulcers, perforation and bleeding in the stomach and gut (celecoxib SmPC).

Symptoms

Gum bleeding, epistaxis, bruising without trauma, blood in urine or stool, menorrhagia. Severe cases include gastrointestinal or intracranial haemorrhage. Risk rises in patients over 65 and with prior peptic ulcer disease.

Management

High doses of non-steroidal anti-inflammatory drugs (NSAIDs) are contraindicated with warfarin (warfarin SmPC). At usual doses, celecoxib is combined with warfarin with caution (celecoxib SmPC). Monitor prothrombin time (international normalised ratio, INR) carefully, especially in the first days after starting celecoxib and after any dose change (celecoxib SmPC), and watch for signs of bleeding (US celecoxib label, FDA). The risk of gastrointestinal bleeding is higher in older patients, in those with a history of ulcer or bleeding, with concomitant aspirin, other NSAIDs or glucocorticoids, and with alcohol use (celecoxib SmPC). Celecoxib is contraindicated in active peptic ulcer or gastrointestinal bleeding (celecoxib SmPC).

Mechanism

Warfarin blocks vitamin K-dependent clotting factor synthesis; clopidogrel inhibits platelet aggregation via the P2Y12 receptor. The two antithrombotic mechanisms add up.

Symptoms

Gum bleeding, epistaxis, bruising without trauma, blood in urine or stool, menorrhagia. Severe cases include gastrointestinal or intracranial haemorrhage. Risk rises in patients over 65 and with prior peptic ulcer disease.

Management

Combine only for clear indications (e.g., atrial fibrillation plus coronary stent). Limit dual therapy to 1–6 months depending on risk. Keep INR at the lower end of the target range (2.0–2.5) and provide PPI cover with pantoprazole.

Mechanism

Trimethoprim in co-trimoxazole inhibits CYP2C9. INR rises 50-100% within 3-5 days. High bleeding risk in elderly.

Symptoms

Gum and nose bleeding, petechiae, haematuria within 5-10 days.

Management

If possible – alternative antibiotic. If co-trimoxazole needed – cut warfarin by 30% and check INR on days 3 and 7.

Mechanism

Diclofenac (NSAID) inhibits platelet aggregation, injures the gastric mucosa, and weakly displaces warfarin from albumin binding. Additive GI bleeding risk independent of INR.

Symptoms

Gum bleeding, epistaxis, bruising without trauma, blood in urine or stool, menorrhagia. Severe cases include gastrointestinal or intracranial haemorrhage. Risk rises in patients over 65 and with prior peptic ulcer disease.

Management

Avoid chronic diclofenac on warfarin. For analgesia, choose paracetamol (up to 2 g/day for short courses). For chronic NSAID needs, use a selective COX-2 inhibitor (celecoxib) under pantoprazole cover.

Mechanism

Duloxetine is a serotonin-norepinephrine reuptake inhibitor (SNRI). It depletes the platelet serotonin pool, impairing primary haemostasis. On warfarin, upper GI bleeding risk roughly doubles.

Symptoms

Gum bleeding, epistaxis, bruising without trauma, blood in urine or stool, menorrhagia. Severe cases include gastrointestinal or intracranial haemorrhage. Risk rises in patients over 65 and with prior peptic ulcer disease.

Management

The combination is acceptable with coagulation monitoring. Check the INR more often than usual after duloxetine is started or stopped, until values are stable. Tell the patient to report bruising without injury, bleeding gums, black stools or blood in the urine. Bleeding risk rises further if NSAIDs or aspirin are added, and the duloxetine label calls for caution in that case.

Mechanism

Fluvoxamine inhibits CYP2C19 and CYP1A2 – minor warfarin clearance routes. INR rises within 1–2 weeks of co-administration. Fluvoxamine (an SSRI) also depletes the platelet serotonin pool, impairing primary haemostasis.

Symptoms

Gum bleeding, epistaxis, bruising without trauma, blood in urine or stool, menorrhagia. Severe cases include gastrointestinal or intracranial haemorrhage. Risk rises in patients over 65 and with prior peptic ulcer disease.

Management

After starting fluvoxamine, check INR at 1 and 2 weeks, then monthly. If INR rises above target, reduce warfarin by 10–20%. Alternative antidepressants without CYP1A2/CYP2C19 effect: sertraline or escitalopram with pantoprazole cover.

Mechanism

Gemfibrozil is highly bound to plasma proteins and displaces warfarin from that binding; in vitro studies additionally show inhibition of CYP2C9, the enzyme that metabolises warfarin. Anticoagulant effect intensifies.

Symptoms

Bleeding gums, nosebleeds, bruising after minor knocks, blood in urine and stool. Intracranial haemorrhage carries the greatest danger: sudden headache, limb weakness, speech disturbance.

Management

The FDA label requires reducing the warfarin dose and keeping prothrombin time at the target level. INR is measured frequently until it stabilises, and monitoring repeats with every change in the gemfibrozil dose.

Mechanism

The European ginkgo monograph notes that concomitant use with anticoagulants increases bleeding risk and that the product should only be taken after consulting a doctor in anticoagulant treatment or bleeding tendency. Ginkgolides inhibit platelet-activating factor, so the effect adds to warfarin at a different step of haemostasis.

Symptoms

Bruises from minimal knocks, gums bleeding when brushing, repeated nosebleeds. Spontaneous intracranial and ocular haemorrhage have been reported with ginkgo. The INR may stay in range, because the platelet pathway does not show up in that measure.

Management

Avoid the combination unless the prescriber has explicitly indicated it. The monograph also advises stopping ginkgo two weeks before planned surgery. If the patient takes it for memory or tinnitus, it helps to know that efficacy in those indications is disputed, while the bleeding risk added to warfarin is concrete.

Mechanism

St. John's wort is a potent inducer of the CYP3A4 enzyme (and of CYP2C9 and P-glycoprotein). After 1–2 weeks it speeds up the breakdown of Warfarin and lowers its blood level, weakening its effect.

Symptoms

Drop in international normalized ratio (INR), weaker anticoagulation, risk of thrombosis and thromboembolism.

Management

Do not combine. If St. John's wort was already taken, stop it and monitor INR; the warfarin dose may need lowering after the herb is stopped.

Mechanism

Ibuprofen and warfarin compete for plasma protein (albumin) binding. The free warfarin fraction rises and its anticoagulant effect intensifies. In parallel, ibuprofen reversibly inhibits platelet aggregation and damages the gastric mucosa – two additional bleeding mechanisms. INR may rise modestly, but gastrointestinal bleeding risk increases independently of INR.

Symptoms

Black or tarry stools (melena), vomiting blood or coffee-ground material, epigastric pain. Less severe presentations: gum bleeding, epistaxis, bruising. Risk multiplies in patients with a history of peptic ulcer disease.

Management

For analgesia in a warfarinised patient, paracetamol is preferred (up to 2 g/day, short courses). If chronic NSAID therapy is needed, switch to a selective COX-2 inhibitor (celecoxib) with proton pump inhibitor cover and frequent INR monitoring. Topical NSAIDs (gels, creams) deliver minimal systemic absorption and are acceptable.

Mechanism

Thyroid hormones accelerate catabolism of vitamin K-dependent clotting factors. When levothyroxine starts or its dose increases, warfarin sensitivity rises and INR climbs within 1–4 weeks.

Symptoms

Gum bleeding, epistaxis, bruising without trauma, blood in urine or stool. Severe cases include gastrointestinal or intracranial haemorrhage.

Management

When levothyroxine starts or is escalated, check INR at 1, 2, and 4 weeks and titrate warfarin (typically a 10–25% dose reduction). Once thyroid function stabilises, warfarin requirements stabilise too.

Mechanism

Meloxicam (preferential COX-2 NSAID) inhibits platelet aggregation less than non-selective NSAIDs but still injures the gastric mucosa. On warfarin, bleeding risk rises moderately.

Symptoms

Gum bleeding, epistaxis, bruising without trauma, blood in urine or stool, menorrhagia. Severe cases include gastrointestinal or intracranial haemorrhage. Risk rises in patients over 65 and with prior peptic ulcer disease.

Management

When unavoidable, use the minimum effective meloxicam dose for short courses under pantoprazole cover. Check INR at 1 week. For chronic needs, celecoxib or paracetamol are preferable.

Mechanism

Menaquinone is vitamin K just as phylloquinone is, with a longer plasma half-life. It antagonises warfarin by the same mechanism, and its presence in calcium plus vitamin D bone complexes means it enters the regimen without the patient registering it as vitamin K.

Symptoms

INR falling below range in the weeks after starting a bone complex, with no change in diet or warfarin dose.

Management

Check the composition of any bone or joint complex before taking it with warfarin: K2 often appears in the small print. If it is already in use, do not stop it abruptly; check the INR more frequently while the decision is made with the prescriber.

Mechanism

Metronidazole stereospecifically inhibits CYP2C9, the clearance route for S-warfarin. INR may double within 3–7 days of co-administration.

Symptoms

Gum bleeding, epistaxis, bruising without trauma, blood in urine or stool, menorrhagia. Severe cases include gastrointestinal or intracranial haemorrhage. Risk rises in patients over 65 and with prior peptic ulcer disease.

Management

For short metronidazole courses, empirically reduce warfarin by 20–30% and check INR 3–4 days after starting and 7 days after stopping the antibiotic. For prolonged therapy, titrate warfarin to INR.

Mechanism

Naproxen (NSAID) inhibits platelet aggregation and injures the gastric mucosa. On warfarin, GI bleeding risk multiplies, particularly in older patients and with prior peptic ulcer disease.

Symptoms

Gum bleeding, epistaxis, bruising without trauma, blood in urine or stool, menorrhagia. Severe cases include gastrointestinal or intracranial haemorrhage. Risk rises in patients over 65 and with prior peptic ulcer disease.

Management

Avoid chronic naproxen on warfarin. For analgesia, choose paracetamol. For chronic NSAID needs, use a selective COX-2 inhibitor (celecoxib) under pantoprazole cover.

Mechanism

Warfarin works precisely by antagonising vitamin K: it blocks the recycling that activates factors II, VII, IX and X. Giving vitamin K1 reverses the drug directly. NIH ODS describes this as a serious and potentially dangerous interaction and stresses that consistency is what matters: sudden changes in intake, in either direction, destabilise the INR.

Symptoms

Adding vitamin K drops the INR below range and thrombotic risk returns: leg pain and swelling, sudden breathlessness, neurological deficit. Stopping it abruptly does the reverse, with a high INR, bleeding gums, bruising and dark urine.

Management

Do not add vitamin K to warfarin therapy on your own. Anyone already taking it should not stop abruptly: the aim is steady intake, leafy greens included, with more frequent INR checks during the first weeks of any change. Vitamin K1 as an antidote in overdose is handled in hospital and has nothing to do with the daily supplement.

Mechanism

Prednisolone (a systemic glucocorticoid) raises fibrinogen and clotting factors but can also alter warfarin metabolism through liver enzymes. The effect is unpredictable: INR may either drop or rise.

Symptoms

If INR drops: thromboembolic risk (deep vein thrombosis, stroke in atrial fibrillation). If INR rises: gum bleeding, epistaxis, bruising, GI bleeding (amplified by glucocorticoid-induced gastric mucosa injury).

Management

When prednisolone starts, is escalated, or is withdrawn, check INR at 1 week, then every 2 weeks until stable. Titrate warfarin to INR. PPI cover with pantoprazole is mandatory.

Mechanism

Propafenone inhibits CYP2C9 (S-warfarin metabolism). INR rises 25-50%.

Symptoms

Gum and nose bleeding, petechiae, haematuria within 5-10 days.

Management

Check INR 5-7 days after propafenone start. Warfarin usually reduced by 25%.

Mechanism

Rifampicin induces CYP2C9, CYP3A4 and CYP1A2 – all main warfarin metabolism pathways. Warfarin requirement rises 2–3 fold. After rifampicin withdrawal sensitivity returns over 2–4 weeks, with bleeding risk if the dose is not titrated back down.

Symptoms

During use: subtherapeutic INR, thrombosis risk. After rifampicin withdrawal: sudden INR rise, bleeding risk.

Management

Monitor INR twice weekly for the first 4 weeks of rifampicin; increase warfarin by 25–100% as needed. After rifampicin withdrawal, frequent INR monitoring and downward titration.

Mechanism

Allopurinol weakly inhibits CYP2C9 and reduces metabolism of the more active S-warfarin enantiomer. INR may rise by 0.5–1.

Symptoms

Gum bleeding, epistaxis, bruising without trauma, black or tarry stools, haematuria. Higher risk in older patients, prior peptic ulcer disease, and chronic kidney disease.

Management

On allopurinol initiation or dose increase, check INR at 1 and 2 weeks. If outside target, reduce warfarin by 10%.

Mechanism

NIH ODS describes two routes: vitamin E inhibits platelet aggregation and antagonises vitamin K-dependent clotting factors. With anticoagulants, high doses raise bleeding risk, especially when vitamin K intake is low. The exact clinically relevant amount is not established, though it probably exceeds 400 IU per day.

Symptoms

Bleeding gums, nosebleeds, bruising. The risk rises in someone who eats few leafy greens and takes high-dose vitamin E on their own.

Management

The vitamin E label advises against 400 IU a day or more in general, except in specific disorders. With warfarin, high vitamin E doses (300 mg a day or more) can prolong prothrombin time, so prothrombin time is monitored and the warfarin dose adjusted if needed (vitamin E SmPC). When vitamin E is started or stopped, check coagulation more often: the Spanish warfarin SmPC requires close coagulation control whenever a concomitant medicine is started or stopped, and the US label (FDA) asks for more frequent INR monitoring.

Mechanism

Amitriptyline is metabolised via CYP2C9 (a minor route) – weak competition with warfarin is possible. Mild INR rises have been reported when adding a TCA.

Symptoms

Mostly no changes. In some patients: a 10–15% INR rise 2–4 weeks after starting amitriptyline.

Management

After starting amitriptyline, check INR at 2 and 4 weeks, then resume routine monitoring. If INR rises above target, reduce warfarin by 5–10%. Alternative antidepressants without CYP2C9 dependence: sertraline or agomelatine.

Mechanism

Amoxicillin may reduce gut flora vitamin K synthesis – INR mildly rises. Effect usually moderate.

Symptoms

INR above target, occasional bleeding.

Management

Check INR on day 5 if course longer than 5 days.

Mechanism

The US warfarin label (FDA) lists atorvastatin among inhibitors of CYP3A4, the enzyme that clears the R-enantiomer of warfarin, so in theory it could increase the warfarin effect. In practice, in a clinical study in patients on long-term warfarin, atorvastatin 80 mg a day slightly shortened prothrombin time, by about 1.7 seconds over the first 4 days, and it returned to normal by day 15. Clinically significant interactions have been reported only very rarely (atorvastatin SmPC). The US atorvastatin label also describes no clinically significant effect on prothrombin time.

Symptoms

Gum bleeding, epistaxis, bruising without trauma, blood in urine or stool, menorrhagia. Severe cases include gastrointestinal or intracranial haemorrhage. Risk rises in patients over 65 and with prior peptic ulcer disease.

Management

Measure prothrombin time (INR) before starting atorvastatin and often enough early in treatment to make sure there is no significant change; after that, monitoring follows the usual warfarin schedule. Repeat this if the atorvastatin dose is changed or the drug is stopped (atorvastatin SmPC). The FDA also advises close INR monitoring when a CYP3A4 inhibitor is started, stopped or changed in dose (US warfarin label).

Mechanism

Azithromycin suppresses vitamin K-producing gut flora, raising INR. Bleeding cases have been reported.

Symptoms

Gum bleeding, epistaxis, bruising without trauma, black or tarry stools. Higher risk in older patients and prior peptic ulcer disease.

Management

For short azithromycin courses (3–5 days), check INR 3–5 days after start and 5 days after stopping the antibiotic. If INR rises above target, reduce warfarin by 10–20%. Alternative antibiotics: a cephalosporin (minimal gut flora effect).

Mechanism

Ciprofloxacin inhibits CYP1A2 and CYP3A4, the enzymes that clear the less potent of the two forms of warfarin (US warfarin label, FDA), and increases the anticoagulant effect. How far the international normalised ratio (INR) will rise is hard to predict: the contribution of ciprofloxacin depends on the infection itself, the patient's age and general condition (ciprofloxacin SmPC). Infections and changes in gut flora also alter the response to warfarin on their own (warfarin SmPC).

Symptoms

Gum bleeding, epistaxis, bruising without trauma, blood in urine or stool, menorrhagia. Severe cases include gastrointestinal or intracranial haemorrhage. Risk rises in patients over 65 and with prior peptic ulcer disease.

Management

The combination is not prohibited but needs frequent monitoring. Check the international normalised ratio (INR) frequently during the ciprofloxacin course and shortly after it ends (ciprofloxacin SmPC; US ciprofloxacin label, FDA). Monitor coagulation closely whenever a drug is added to or withdrawn from warfarin therapy (warfarin SmPC); the FDA also advises close INR monitoring when any antibiotic is started or stopped (US warfarin label). Adjust the warfarin dose to the INR if needed (US warfarin label). Ask the patient to report bleeding gums, nosebleeds, bruising, red or dark brown urine and black stools straight away (US warfarin label).

Mechanism

Clarithromycin is one of the enzyme inhibitors that slow the liver metabolism of warfarin and increase its anticoagulant effect and bleeding risk (warfarin SmPC). The US Food and Drug Administration (FDA) lists clarithromycin as an inhibitor of CYP3A4, which clears one of the two warfarin isomers (warfarin FDA label). Serious bleeding and marked rises in the international normalised ratio (INR) and prothrombin time can occur with this combination (clarithromycin SmPC).

Symptoms

Bleeding gums, nosebleeds, bruising and pinpoint skin bleeds, blood in the urine, black stools, heavy periods. In severe cases, gastrointestinal or intracranial bleeding.

Management

The combination can be used with frequent coagulation checks. Check the international normalised ratio (INR) and prothrombin time frequently throughout the clarithromycin course (clarithromycin SmPC). The US Food and Drug Administration (FDA) advises close INR monitoring when any antibiotic is started or stopped (warfarin FDA label). The European Society of Cardiology (ESC) lists macrolides among drugs that call for careful INR monitoring in patients on vitamin K antagonists (ESC 2024). Adjust the warfarin dose according to the INR (warfarin SmPC). Ask the patient to report any signs of bleeding (warfarin FDA label).

Mechanism

Dexamethasone is a weak CYP3A4 inducer and a potential weak CYP2C9 inhibitor. Effect on INR is unpredictable and usually mild: INR may fall in some patients and rise in others.

Symptoms

Gum bleeding, epistaxis, bruising without trauma, black or tarry stools, haematuria. Higher risk in older patients, prior peptic ulcer disease, and chronic kidney disease.

Management

For short dexamethasone courses (3–5 days), no special monitoring needed. For longer therapy, check INR at one week, then every 2 weeks. Beyond the INR effect, glucocorticoids add gastrointestinal bleeding risk via mucosal ulcerogenic action.

Mechanism

Fluconazole moderately inhibits CYP2C9, which clears the more potent S-warfarin, and also CYP3A4 (fluconazole and warfarin labels). The warfarin effect increases: prothrombin time was prolonged up to 2-fold during co-administration (fluconazole SmPC). Bleeding events have been reported with this combination.

Symptoms

Bruising, nosebleeds, blood in the urine, black tarry stools and gastrointestinal bleeding, with a prolonged prothrombin time and a rising INR.

Management

The combination is not contraindicated but needs frequent coagulation checks. While both drugs are taken, monitor prothrombin time carefully and adjust the warfarin dose if needed (fluconazole SmPC). The Spanish warfarin SmPC requires close coagulation control whenever a concomitant medicine is added or stopped, and the US warfarin label (FDA) asks for close INR monitoring when any antifungal is started or stopped. So check the INR more often after the fluconazole course ends as well.

Mechanism

EPA displaces arachidonic acid in the platelet membrane and shifts synthesis towards thromboxane A3, far less aggregatory than A2. The effect on bleeding time is real but moderate and dose-dependent: it appears mainly above 3 g daily of EPA plus DHA, not with the usual 1 g capsule.

Symptoms

Bruises come slightly more easily and bleeding lasts longer after cuts or a dental extraction. At usual supplement doses most patients notice no change.

Management

There is no need to stop omega-3 routinely. Keep the dose steady, declare it to the prescriber and monitor INR as usual. If the regimen exceeds 3 g daily or antiplatelets are added, review the supplement dose before the anticoagulant's. Before planned surgery, discuss it with the surgical team.

Mechanism

Paracetamol at doses above 3 g/day for more than 7 days raises INR by 1–1.5 via hepatocyte suppression and weak effect on warfarin metabolism. A single 0.5–1 g dose is safe.

Symptoms

Gum bleeding, epistaxis, bruising without trauma, black or tarry stools, haematuria. Higher risk in older patients, prior peptic ulcer disease, and chronic kidney disease.

Management

Paracetamol remains the analgesic and antipyretic of choice on warfarin, but for chronic use cap at 2 g/day. For full-dose courses longer than 4 days: check INR after a week; if above target, reduce warfarin by 10%.

Mechanism

Complex mechanism. Ritonavir inhibits CYP3A4 but simultaneously induces CYP1A2, CYP2C9, and CYP2C19 (long-term effect). The net effect on warfarin varies: in some patients INR rises, in others it falls.

Symptoms

If INR rises: gum bleeding, epistaxis, bruising without trauma, blood in urine or stool. If INR drops: thromboembolic risk (deep vein thrombosis, stroke in atrial fibrillation).

Management

The combination can be used with closer coagulation monitoring. The US ritonavir label (FDA) recommends frequent INR checks at the start of co-administration. The European ritonavir SmPC asks for monitoring of anticoagulation parameters both at antiretroviral doses and at the low dose used to boost other protease inhibitors. Adjust the warfarin dose to the INR. The Spanish warfarin SmPC requires close coagulation control whenever a concomitant medicine is added or stopped, so check the INR more often after ritonavir is stopped as well (warfarin SmPC).

Mechanism

Sertraline, like other selective serotonin reuptake inhibitors (SSRIs), alters platelet function and raises bleeding risk on its own, from bruising and nosebleeds to life-threatening haemorrhage (US sertraline label, FDA). With warfarin these risks add up (US sertraline label). Sertraline 200 mg a day also produced a small but statistically significant increase in prothrombin time, which in rare cases can change the international normalised ratio (INR) (sertraline SmPC).

Symptoms

Gum bleeding, epistaxis, bruising without trauma, blood in urine or stool, menorrhagia. Severe cases include gastrointestinal or intracranial haemorrhage. Risk rises in patients over 65 and with prior peptic ulcer disease.

Management

The combination is acceptable with monitoring. Check prothrombin time (international normalised ratio, INR) carefully when sertraline is started or stopped (sertraline SmPC). The US Food and Drug Administration (FDA) also advises careful INR monitoring in patients on warfarin (US sertraline label) and close monitoring for signs of bleeding (US warfarin label). Take particular care in patients with a history of bleeding disorders (sertraline SmPC). Warn the patient about the higher bleeding risk and ask them to mention any over-the-counter medicines that add to it, such as aspirin and non-steroidal anti-inflammatory drugs (US sertraline label).

Mechanism

Simvastatin weakly inhibits CYP3A4 and partly displaces warfarin from protein binding. INR may rise by 0.5–1 in the first 2 weeks.

Symptoms

Gum bleeding, epistaxis, bruising without trauma, black or tarry stools, haematuria. Higher risk in older patients, prior peptic ulcer disease, and chronic kidney disease.

Management

On simvastatin initiation, check INR at 1 and 2 weeks, then per usual schedule. If INR rises, reduce warfarin by 10%. Alternatives: pravastatin or rosuvastatin (minimal warfarin effect).

Mechanism

Tramadol indirectly affects CYP2C9-mediated warfarin metabolism via CYP2D6. INR increases of 1–2 points have been reported, especially in older patients.

Symptoms

Gum bleeding, epistaxis, bruising without trauma, black or tarry stools, haematuria. Higher risk in older patients, prior peptic ulcer disease, and chronic kidney disease.

Management

On tramadol start, check INR at 5–7 days. For pronounced pain, alternatives: paracetamol up to 2 g/day or a short tramadol course at 50 mg at night. High-dose opioids and NSAIDs are less preferred.