Vitamin K antagonist anticoagulants
ATC code: B01AA03 (Warfarin)
Inhibits hepatic vitamin K epoxide reductase (VKORC1), impairing γ-carboxylation of clotting factors II, VII, IX, X and proteins C/S. Anticoagulant effect develops over 36–72 h. Narrow therapeutic window (INR 2.0–3.0 for most indications) demands routine laboratory monitoring. Metabolized by CYP2C9; CYP2C9 and VKORC1 polymorphisms account for substantial interindividual dose variability.
First line
Warfarin reduces stroke risk in atrial fibrillation by 64 % compared to placebo. Target INR 2.0–3.0. Direct oral anticoagulants (DOACs) have largely replaced warfarin as first choice in non-valvular AF. Warfarin remains first-line in valvular AF, particularly with mitral stenosis or mechanical valves.
First line
Warfarin is the only approved anticoagulant for mechanical heart valve prostheses. DOACs are contraindicated – the RE-ALIGN trial (dabigatran) was stopped early due to increased thromboembolism and bleeding. Target INR depends on valve type and position: 2.5–3.5 for mitral and older aortic valves, 2.0–3.0 for modern bileaflet aortic valves.
First line
Warfarin is a well-established anticoagulant for treatment and secondary prevention of DVT and PE. Target INR 2.0–3.0. Duration depends on context: 3 months for provoked VTE, indefinitely for unprovoked or recurrent. DOACs are the preferred alternative, but warfarin remains an option in antiphospholipid syndrome and when DOACs are unavailable.
The drug is promoted for these uses outside international guidelines. Each entry below is analyzed against AEMPS, FDA, EMA, Cochrane and major RCTs.
Not recommended
Warfarin is an indirect anticoagulant, a vitamin K antagonist. It is prescribed for atrial fibrillation with stroke risk, venous thromboembolism (DVT/PE), and prosthetic heart valves ( 2024, 2023). In anti-aging circles, warfarin is taken for blood thinning outside these indications. In healthy adults, no clinical studies of prophylactic warfarin for primary CV prevention exist, and such prescription is dangerous. The drug has serious risks: narrow therapeutic window (requires regular INR monitoring), multiple food and drug interactions (especially vitamin K and CYP2C9 substrates), elevated hemorrhagic stroke risk in older adults, and bleeding at any site. If warfarin was prescribed for blood thinning outside atrial fibrillation, thrombosis, or a prosthetic valve, consider seeking a second opinion.
INR effect is unpredictable: acute alcohol intoxication raises INR, chronic abuse via CYP induction lowers it. Binge drinking – high bleeding risk. Stable moderate intake acceptable with counseling.
Source: FDA: Coumadin (warfarin) prescribing information (2021)
Vitamin K is a warfarin antagonist: it restores synthesis of vitamin-K-dependent clotting factors (II, VII, IX, X). Sudden increases in dietary vitamin K (leafy greens) lower INR. Maintain stable intake without sharp variations.
Source: AHA/ACC: 2020 ACC/AHA Guideline for the Management of Patients With Valvular Heart Disease (2020)
37 pairs found. Sorted from critical to minor.
Mechanism
Aspirin irreversibly blocks platelet cyclooxygenase and reduces platelet aggregation. Warfarin blocks vitamin K-dependent clotting factor synthesis. Two different antithrombotic mechanisms add up. Aspirin also damages the gastric mucosa, raising bleeding risk independently of coagulation parameters.
Symptoms
Gum bleeding, epistaxis, black or tarry stools (melena), vomiting blood or coffee-ground material. Severe cases include fatal intracranial haemorrhage. Risk multiplies in patients over 75 and with prior peptic ulcer disease.
Management
The combination is acceptable only for strict indications: after coronary stenting, mechanical heart valves, acute coronary syndrome. Limit triple/dual antithrombotic therapy to 1–6 months based on risk. Aspirin dose: 75–100 mg/day. PPI cover (pantoprazole) is mandatory. Check INR every 2–4 weeks.
Sources
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.
Sources
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
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
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.
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 strict indications: after coronary stenting, mechanical heart valves, acute coronary syndrome. Limit triple/dual antithrombotic therapy to 1–6 months based on risk. Aspirin 75–100 mg/day. Mandatory pantoprazole cover; check INR every 2–4 weeks.
Mechanism
Atorvastatin weakly inhibits CYP3A4 and CYP2C9 – the warfarin clearance route. Some patients show a 20–30% INR rise within 2–4 weeks; a small share experience clinically significant effects.
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. After starting atorvastatin, check INR at 2 and 4 weeks, then resume routine monitoring. If INR rises above target, reduce warfarin by 10–20%.
Sources
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 is metabolised by CYP2C9 – the same route as S-warfarin (the more active form). Enzyme competition raises INR. Celecoxib also moderately injures the gastric mucosa.
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 when chronic NSAID therapy is needed. Check INR at 1 and 2 weeks after starting celecoxib, then monthly. PPI cover with pantoprazole is mandatory.
Mechanism
Ciprofloxacin blocks CYP1A2 and, to a lesser extent, CYP3A4 – warfarin clearance routes. It also suppresses vitamin K-producing gut flora. INR rises 1.5- to 2-fold within 5–7 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
During ciprofloxacin therapy, empirically reduce warfarin by 20%; check INR 3–5 days after starting and 7 days after stopping the antibiotic. Alternatives: cephalosporin or nitrofurantoin (minimal CYP impact).
Mechanism
Clarithromycin inhibits CYP3A4 (a minor warfarin clearance route) and suppresses vitamin K-producing gut flora. INR rises 1.5- to 2-fold within 3–5 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
During clarithromycin therapy, empirically reduce warfarin by 20–30% and check INR 3–4 days after starting and 7 days after stopping the antibiotic. Alternative: azithromycin (minimal CYP3A4 and gut-flora impact).
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.
Sources
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. 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 and sulfamethoxazole (the components of co-trimoxazole) both inhibit CYP2C9, the main S-warfarin clearance route. Co-trimoxazole additionally displaces warfarin from plasma protein binding. INR may rise 2- to 4-fold within 3–7 days.
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 a short co-trimoxazole course (up to 7 days), empirically reduce warfarin by 30–50% and check INR every 3 days. For urinary tract infections in warfarin-treated patients, prefer nitrofurantoin or fosfomycin (minimal CYP2C9 impact).
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. PPI cover with pantoprazole is mandatory. Check INR at 2 and 4 weeks after starting duloxetine, then monthly. Alternative antidepressants with minimal platelet effect: mirtazapine or agomelatine.
Mechanism
Fluconazole inhibits CYP2C9, the main enzyme of S-warfarin (the more active isomer). INR rises to 8–12 with risk of major bleeding: gastrointestinal, intracranial, retroperitoneal.
Symptoms
By days 3–5: gum and nose bleeding, petechiae, haematuria, bloody stool. INR above 6 without clinical bleeding still requires action.
Management
If fluconazole is required, reduce warfarin by 25–50% from day 1 and check INR on days 3 and 7. Short courses (3 single doses) – INR check on day 5. After fluconazole withdrawal, retitrate warfarin upward.
Sources
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.
Sources
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.
Sources
Mechanism
Ketorolac is the most potent available NSAID. It inhibits platelet aggregation and strongly injures the gastric mucosa. On warfarin, massive GI bleeding risk multiplies. Toradol Section 4.3 lists anticoagulants as contraindications.
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 prescribed. For analgesia on warfarin, use paracetamol (up to 2 g/day), metamizole, or a short opioid course. Ketorolac courses never exceed 5 days under any circumstances.
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.
Sources
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
Metronidazole stereospecifically inhibits CYP2C9, the clearance route for S-warfarin (the more active stereoisomer). 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 (5–7 days), empirically reduce warfarin by 20–30% and check INR 3–4 days after starting and 7 days after stopping the antibiotic. For prolonged metronidazole therapy, titrate warfarin to INR.
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
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
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.
Sources
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
When ritonavir starts, is escalated, or is withdrawn, check INR at 1, 2, and 4 weeks and titrate warfarin to effect. Alternative anticoagulant in HIV-positive patients: low molecular weight heparin for short courses (no CYP interactions).
Mechanism
Sertraline (SSRI) blocks serotonin reuptake into platelets, depleting the platelet serotonin pool and impairing primary haemostasis. Even with a stable INR, GI bleeding risk roughly doubles in warfarin-treated patients.
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. PPI cover with pantoprazole and more frequent INR checks (every 2–4 weeks in the first month) are advisable. Alternative antidepressants with minimal platelet effect: mirtazapine or agomelatine.
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
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 suppresses vitamin K-producing gut flora and enhances warfarin anticoagulation. Effect is unpredictable: INR may stay flat or rise by 1–2.
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 amoxicillin courses of 5–7 days, check INR 3–5 days after start and one week after stopping. If INR drifts outside target, adjust warfarin by 10–20%. Do not stop the antibiotic — complete the full course for the infection.
Sources
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
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
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
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).
Sources
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.
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FDA category X. Warfarin crosses the placenta and causes warfarin embryopathy: nasal hypoplasia, stippled chondral calcification, CNS abnormalities. Particularly dangerous at weeks 6–12. If anticoagulation is needed during pregnancy, switch to low-molecular-weight heparin.
Warfarin is excreted in breast milk in negligible amounts. Per AAP, breastfeeding is compatible with warfarin use. Infant INR monitoring is recommended in the first weeks.
Reference information, not a clinical decision. Discuss feeding pauses or changes with your physician or an IBCLC.
Warfarin is evaluated for the following indications with varying evidence strength: Venous thromboembolism (evidence tier A), Mechanical heart valve (evidence tier A), Atrial fibrillation (evidence tier A). See the full indication matrix with dosing and citations above on this page.
Common side effects of Warfarin (≥ 1 in 100): Bleeding (epistaxis, gingival bleeding, ecchymoses, hematuria), Dose sensitivity to dietary and drug interactions. See the Safety section for uncommon and serious reactions.
FDA category X. FDA category X. Warfarin crosses the placenta and causes warfarin embryopathy: nasal hypoplasia, stippled chondral calcification, CNS abnormalities. Particularly dangerous at weeks 6–12. If anticoagulation is needed during pregnancy, switch to low-molecular-weight heparin.
Warfarin is excreted in breast milk in negligible amounts. Per AAP, breastfeeding is compatible with warfarin use. Infant INR monitoring is recommended in the first weeks.
Warfarin is contraindicated in: Active bleeding; Pregnancy (teratogenic – category X); Severe uncontrolled hypertension; Recent CNS or ophthalmic surgery; Inability to monitor INR regularly. Full list in the Safety section.