Ciprofloxacin

Mechanism

The liver breaks down agomelatine mainly through the enzyme CYP1A2, which handles 90% of its metabolism. The remaining 10% goes through the enzymes CYP2C9 and CYP2C19. Ciprofloxacin inhibits CYP1A2, but there are no clinical data on this exact pair. The closest guide is fluvoxamine, a strong CYP1A2 inhibitor, which raised agomelatine exposure 60-fold (range 12 to 412). The ciprofloxacin label expects a similar effect. The agomelatine label lists ciprofloxacin next to fluvoxamine as a strong CYP1A2 inhibitor, while the ciprofloxacin label calls it a moderate one. Both drugs can damage the liver: hepatitis and liver failure have been reported with agomelatine, and liver necrosis with ciprofloxacin.

Symptoms

Common adverse effects of agomelatine include headache, dizziness, drowsiness, nausea, anxiety and raised levels of the liver enzymes alanine aminotransferase (ALT) and aspartate aminotransferase (AST). Reported signs of agomelatine overdose include upper abdominal (epigastric) pain, drowsiness, tiredness, agitation, anxiety, tension, dizziness, cyanosis and general malaise. With agomelatine, liver injury mostly appears in the first months of treatment. It shows as dark urine, pale stools, yellow skin or eyes, pain in the upper right abdomen and sudden, unexplained, prolonged fatigue. The ciprofloxacin label adds loss of appetite, itching and abdominal tenderness to the signs of liver disease.

Management

The combination is contraindicated. The agomelatine label forbids use with strong inhibitors of the enzyme CYP1A2 and names ciprofloxacin (agomelatine label, European Medicines Agency, EMA, and Spanish Agency of Medicines and Medical Devices, AEMPS). Do not take ciprofloxacin and agomelatine at the same time. The labels give no waiting period for switching from one drug to the other. If the two have been taken together, monitor the patient closely for signs of agomelatine overdose (ciprofloxacin label, AEMPS). There is no specific antidote for agomelatine: treat the symptoms and monitor the patient, preferably in a specialised setting (agomelatine label, EMA). Both drugs can damage the liver, and the agomelatine label asks for caution and close follow-up when it is combined with such drugs (agomelatine label, EMA). During agomelatine treatment, check liver function before the start, at about 3, 6, 12 and 24 weeks and then when clinically indicated. Repeat the tests within 48 hours if transaminases rise (agomelatine label, EMA). Stop agomelatine at once if signs of liver injury appear or transaminases exceed 3 times the upper limit of normal (agomelatine label, EMA). Stop ciprofloxacin at any sign of liver disease (ciprofloxacin label, AEMPS).

Mechanism

Ciprofloxacin is a fluoroquinolone, and its SmPC lists concomitant QT-prolonging drugs among the risk factors that call for caution. The citalopram SmPC lists ongoing treatment with any drug capable of prolonging the QT interval among its contraindications, with no exceptions.

Symptoms

QTc prolongation on ECG, palpitations, dizziness, syncope. The end point is polymorphic ventricular tachycardia of the torsade type progressing to ventricular fibrillation. The risk is higher with hypokalaemia, hypomagnesaemia, bradycardia, decompensated heart failure and in women.

Management

The pair sits in the citalopram SmPC contraindications, so the question is which drug stays, not how far to cut the dose. Ciprofloxacin's cardiac effect is weaker than moxifloxacin's, yet the SSRI ban makes no exception for weaker effects. Urinary tract infections have replacements outside the fluoroquinolones.

Mechanism

Colchicine is cleared through the CYP3A4 enzyme and the P-glycoprotein transporter and has a narrow therapeutic margin (colchicine SmPC). Ciprofloxacin's own labels describe it as a CYP1A2 inhibitor, while the US Food and Drug Administration (FDA) also lists it as a moderate CYP3A4 inhibitor. CYP3A4 inhibition can raise colchicine blood levels and, with them, the risk of toxicity.

Symptoms

Nausea, vomiting, diarrhoea, muscle weakness, paraesthesias. With severe toxicity: myopathy, neuropathy, bone marrow suppression.

Management

The combination is contraindicated by the Spanish colchicine SmPC, which forbids colchicine during use of CYP3A4 or P-glycoprotein inhibitors and for 14 days afterwards (colchicine SmPC). The US Food and Drug Administration (FDA) classifies ciprofloxacin as a moderate CYP3A4 inhibitor (FDA list), so colchicine is not given during a ciprofloxacin course or for 14 days after it. Other colchicine manufacturers require a lower maximum colchicine dose and close monitoring with a moderate CYP3A4 inhibitor (colchicine SmPC), and the US label of colchicine for cardiovascular prevention advises avoiding such drugs in renal or hepatic impairment (colchicine label, FDA). Colchicine itself is contraindicated in severe renal or hepatic impairment (colchicine SmPC). At the first sign of nausea, vomiting, abdominal pain or diarrhoea, colchicine is stopped and the patient needs urgent care (colchicine SmPC).

Mechanism

Ciprofloxacin is a potent CYP1A2 inhibitor, the main duloxetine metabolic route. Duloxetine plasma levels rise 6-fold. Nausea, sedation, and serotonergic effects emerge.

Symptoms

Serotonin syndrome: agitation, confusion, tremor, myoclonus, hyperreflexia. Autonomic features: sweating, tachycardia, hypertension, fever. First signs appear within hours of co-administration.

Management

The AEMPS label for duloxetine lists the combination with ciprofloxacin as contraindicated: ciprofloxacin, a strong CYP1A2 inhibitor, sharply raises duloxetine exposure. Alternative antibiotics without CYP1A2 effect: a cephalosporin, nitrofurantoin, or fosfomycin.

Mechanism

Ciprofloxacin is a fluoroquinolone, and its SmPC lists concomitant QT-prolonging drugs among the risk factors that call for caution. The escitalopram SmPC contraindicates any QT-prolonging drug alongside it: the ban is written broadly, and the classes in brackets are examples.

Symptoms

QTc prolongation on ECG, palpitations, dizziness, syncope. The end point is polymorphic ventricular tachycardia of the torsade type progressing to ventricular fibrillation. The risk is higher with hypokalaemia, hypomagnesaemia, bradycardia, decompensated heart failure and in women.

Management

The pair sits in the escitalopram SmPC contraindications, so the question is which drug stays, not how far to cut the dose. Ciprofloxacin's cardiac effect is weaker than moxifloxacin's, yet the SSRI ban makes no exception for weaker effects. Urinary tract infections have replacements outside the fluoroquinolones.

Mechanism

Additive QT prolongation. Amiodarone strongly blocks IKr potassium channels, ciprofloxacin weakly but dose-dependently. Ciprofloxacin also inhibits CYP1A2 (a minor amiodarone clearance route).

Symptoms

QT prolongation on ECG. Clinically: dizziness, syncope, palpitations. Severe cases progress to polymorphic ventricular tachycardia (torsades de pointes) with risk of ventricular fibrillation and sudden cardiac death. Risk is higher with hypokalaemia, hypomagnesaemia, bradycardia, and ischaemic heart disease.

Management

Avoid where possible. Alternative antibiotic: a cephalosporin (no QT effect). If ciprofloxacin is needed, ECG before start and at day 3, with potassium and magnesium maintained.

Mechanism

Additive QT prolongation. Ciprofloxacin additionally blocks CYP1A2 – an amitriptyline metabolic route. Amitriptyline plasma levels rise.

Symptoms

QT prolongation on ECG. Dizziness, syncope, palpitations. Severe cases: polymorphic ventricular tachycardia (torsades de pointes). Risk is higher with hypokalaemia and hypomagnesaemia.

Management

Avoid the combination. Alternative antibiotics without QT effect: a cephalosporin or nitrofurantoin. Alternative for neuropathic pain instead of TCA: gabapentin or pregabalin.

Mechanism

Both antibiotics prolong the QT interval on the electrocardiogram, and taken together they raise the risk of arrhythmias, including torsades de pointes (azithromycin and ciprofloxacin SmPCs).

Symptoms

QT prolongation on the electrocardiogram (ECG), palpitations, dizziness, fainting. Rarely, torsades de pointes, which can lead to cardiac arrest (azithromycin SmPC). The risk is higher in older patients and women, with hypokalaemia or hypomagnesaemia, bradycardia, heart failure and after myocardial infarction (azithromycin and ciprofloxacin SmPCs).

Management

Avoid the combination: the ciprofloxacin label of the US Food and Drug Administration (FDA) says to avoid ciprofloxacin in patients taking macrolides and other drugs that prolong the QT interval (ciprofloxacin FDA label). The Spanish SmPCs of both antibiotics allow the combination only with caution (azithromycin and ciprofloxacin SmPCs). If the combination cannot be avoided, note that the FDA label also says to avoid ciprofloxacin with known QT prolongation, uncorrected hypokalaemia or hypomagnesaemia, heart failure, myocardial infarction or bradycardia (ciprofloxacin FDA label). Older patients and women are more sensitive to QT prolongation (azithromycin and ciprofloxacin SmPCs).

Mechanism

Fluoroquinolones, including ciprofloxacin, can cause tendinitis and tendon rupture, most often of the Achilles tendon. The risk is higher in older patients, in renal impairment, after solid organ transplantation and in patients who take corticosteroids at the same time (ciprofloxacin SmPC). The US Food and Drug Administration (FDA) label specifies age over 60 and adds strenuous physical activity and previous tendon disorders as risk factors (ciprofloxacin FDA label).

Symptoms

Pain, swelling and inflammation of the Achilles or other tendons, sometimes on both sides, up to tendon rupture. Symptoms can start within 48 hours of starting ciprofloxacin and as late as several months after the course ends (ciprofloxacin SmPC).

Management

The ciprofloxacin SmPC says concomitant use of corticosteroids should be avoided because of the risk of tendinitis and tendon rupture (ciprofloxacin SmPC). It does not specify the route; inhaled budesonide can also have systemic effects, especially at high doses and in long-term use (inhaled budesonide SmPC). Ciprofloxacin is generally not given to patients who have had tendon disorders with a quinolone before (ciprofloxacin SmPC). Warn the patient about the symptoms: at the first sign of tendinitis, such as painful swelling or inflammation, stop ciprofloxacin at once and immobilise the affected limb, and do not use corticosteroids if there are signs of tendinopathy (ciprofloxacin SmPC).

Mechanism

Ciprofloxacin blocks CYP1A2 and lowers seizure threshold itself. Bupropion is a potent proconvulsant. Ciprofloxacin also weakly blocks CYP2B6 (a minor bupropion route).

Symptoms

Seizures (generalised tonic-clonic or focal), agitation, tremor, insomnia. Patients with prior epilepsy or eating disorders are at particular risk.

Management

Avoid the combination, especially in patients with prior epilepsy. Alternative antibiotics: a cephalosporin, nitrofurantoin, or fosfomycin (no effect on seizure threshold).

Mechanism

The ciprofloxacin label describes the mechanism as chelate complex formation and names calcium explicitly. The antibiotic stays trapped in the gut and plasma levels fall short of what the organism requires. Dairy and calcium-fortified juices count the same as the supplement, although calcium from an ordinary diet does not affect absorption significantly.

Symptoms

The infection does not settle on schedule: fever persists beyond 48–72 hours, the urine culture stays positive after the course. The real consequence is not toxicity but treatment failure and selection of resistance.

Management

Ciprofloxacin 1–2 hours before calcium or at least 4 hours after, per the label. During the course the simplest arrangement is to move the supplement to dinner and take the antibiotic in the morning and mid-afternoon. H2-receptor antacids are exempt from this restriction.

Mechanism

Fluoroquinolones, including ciprofloxacin, can cause tendinitis and tendon rupture, most often of the Achilles tendon. The risk is higher in older patients, in renal impairment, after solid organ transplantation and in patients who take corticosteroids at the same time (ciprofloxacin SmPC); the dexamethasone SmPC also lists use with fluoroquinolones as a risk factor for tendinitis and tendon rupture (dexamethasone SmPC). The US Food and Drug Administration (FDA) label specifies age over 60 and adds strenuous physical activity and previous tendon disorders as risk factors (ciprofloxacin FDA label). A second mechanism is metabolic: dexamethasone is cleared by the CYP3A4 enzyme (dexamethasone SmPC), and the FDA classes ciprofloxacin as a moderate CYP3A inhibitor (FDA list), so dexamethasone levels may rise.

Symptoms

Pain, swelling and inflammation of the Achilles tendon, less often of tendons in the shoulder, hand or biceps, sometimes on both sides, up to tendon rupture (ciprofloxacin labels). Symptoms can start within 48 hours of starting ciprofloxacin and as late as several months after the course ends (ciprofloxacin SmPC).

Management

The ciprofloxacin SmPC says concomitant use of corticosteroids should be avoided because of the risk of tendinitis and tendon rupture (ciprofloxacin SmPC). The dexamethasone SmPC also advises avoiding CYP3A inhibitors unless the benefit outweighs the higher risk of systemic corticosteroid adverse effects, and monitoring the patient for them if the combination is used (dexamethasone SmPC); the US Food and Drug Administration (FDA) classes ciprofloxacin as a moderate CYP3A inhibitor (FDA list). Ciprofloxacin is generally not given to patients who have had tendon disorders with a quinolone before (ciprofloxacin SmPC). Warn the patient about the symptoms: at the first sign of tendinitis, such as painful swelling or inflammation, stop ciprofloxacin at once and immobilise the affected limb, and do not use corticosteroids if there are signs of tendinopathy (ciprofloxacin SmPC).

Mechanism

Additive QT prolongation. Dronedarone is on the FDA list of drugs that contraindicate strong QT-prolonging antibiotics. Ciprofloxacin prolongs QT dose-dependently.

Symptoms

QT prolongation on ECG. Clinically: dizziness, syncope, palpitations. Severe cases progress to polymorphic ventricular tachycardia (torsades de pointes) with risk of ventricular fibrillation and sudden cardiac death. Risk is higher with hypokalaemia, hypomagnesaemia, bradycardia, and ischaemic heart disease.

Management

Avoid the combination. Alternative antibiotics: a cephalosporin (no QT effect) or nitrofurantoin for urinary tract infection.

Mechanism

Ciprofloxacin disrupts insulin secretion from pancreatic β-cells and concurrently inhibits CYP2C9 – the glibenclamide metabolic route. Glibenclamide plasma levels rise, intensifying its hypoglycaemic effect. FDA warning was issued in 2018.

Symptoms

Severe hypoglycaemia: sweating, tremor, hunger, tachycardia, confusion; severe cases progress to seizures and coma. Symptoms appear within 1–7 days, especially in older patients and chronic kidney disease.

Management

Avoid the combination. Alternative antibiotics: a cephalosporin or fosfomycin (no CYP2C9 effect and no β-cell impact). If ciprofloxacin is needed, monitor glucose 4 times daily and warn the patient about hypoglycaemia signs.

Mechanism

Dual mechanism: both prolong QT (additively), and ciprofloxacin blocks CYP1A2 – a partial haloperidol clearance route. Haloperidol levels rise.

Symptoms

QT prolongation on ECG. Clinically: dizziness, syncope, palpitations. Severe cases progress to polymorphic ventricular tachycardia (torsades de pointes) with risk of ventricular fibrillation and sudden cardiac death. Risk is higher with hypokalaemia, hypomagnesaemia, bradycardia, and ischaemic heart disease.

Management

Avoid the combination. Alternative antibiotics: a cephalosporin or nitrofurantoin. Alternative antipsychotics with minimal QT effect: olanzapine or aripiprazole.

Mechanism

Iron belongs to the same multivalent cation list in the label. Chelation with fluoroquinolones is among the most pronounced described: the antibiotic's bioavailability falls substantially when the two meet in the stomach.

Symptoms

Cystitis or prostatitis that relapses shortly after the course ends, with the same organism on the follow-up culture.

Management

The same interval applies: 1–2 hours before or 4 hours after. If iron is being taken for anaemia and cannot pause, concentrate it in a single evening dose and spread the antibiotic across the rest of the day. In serious infection the prescriber may prefer a class without this problem.

Mechanism

Magnesium is among the cations the label names outright. Magnesium-containing antacids and heavily buffered tablets fall under the same warning, so the interaction shows up both with the supplement and with the heartburn remedy the patient does not think of as a drug.

Symptoms

Slow or absent clinical response to the antibiotic, with no intolerance signs to point at the cause.

Management

Interval of 1–2 hours before or 4 hours after. Ask directly about antacids and evening magnesium: they rarely come up when the patient is asked what medication they take.

Mechanism

Fluoroquinolones, including ciprofloxacin, can cause tendinitis and tendon rupture, most often of the Achilles tendon. The risk is higher in older patients, in renal impairment, after solid organ transplantation and in patients who take corticosteroids at the same time (ciprofloxacin SmPC). Methylprednisolone itself very rarely causes tendon rupture, for example of the Achilles tendon (methylprednisolone SmPC). The US Food and Drug Administration (FDA) label specifies age over 60 and adds strenuous physical activity and previous tendon disorders as risk factors (ciprofloxacin FDA label). A second mechanism is metabolic: some corticosteroids are cleared by the CYP3A4 enzyme (methylprednisolone SmPC), and the FDA classes ciprofloxacin as a moderate CYP3A inhibitor (FDA list), so methylprednisolone levels may rise.

Symptoms

Pain, swelling and inflammation of the Achilles tendon, less often of tendons in the shoulder, hand or biceps, sometimes on both sides, up to tendon rupture (ciprofloxacin labels). Symptoms can start within 48 hours of starting ciprofloxacin and as late as several months after the course ends (ciprofloxacin SmPC).

Management

The ciprofloxacin SmPC says concomitant use of corticosteroids should be avoided because of the risk of tendinitis and tendon rupture (ciprofloxacin SmPC). The methylprednisolone SmPC also advises avoiding CYP3A inhibitors unless the benefit outweighs the higher risk of systemic corticosteroid adverse effects, and monitoring the patient for them if the combination is used (methylprednisolone SmPC); the US Food and Drug Administration (FDA) classes ciprofloxacin as a moderate CYP3A inhibitor (FDA list). Ciprofloxacin is generally not given to patients who have had tendon disorders with a quinolone before (ciprofloxacin SmPC). Warn the patient about the symptoms: at the first sign of tendinitis, such as painful swelling or inflammation, stop ciprofloxacin at once and immobilise the affected limb, and do not use corticosteroids if there are signs of tendinopathy (ciprofloxacin SmPC).

Mechanism

Additive tendon injury risk. Fluoroquinolones disrupt collagen synthesis in tendon fibres; glucocorticoids suppress repair. FDA carries a boxed warning for tendinitis and Achilles tendon rupture.

Symptoms

Pain and swelling in the Achilles tendon (less often other large tendons). Sudden 'snap' and inability to bear weight indicates rupture. Symptoms typically appear within 1–4 weeks.

Management

Avoid the combination, especially in patients over 60 and athletes. Alternative antibiotics: a cephalosporin or nitrofurantoin (no tendon effect). If ciprofloxacin is needed, limit physical activity for the whole course and 1 month after.

Mechanism

Both drugs prolong the QT interval. Sertraline does so in a concentration-dependent way: at twice the maximum recommended dose the largest mean increase was 10 ms (sertraline label of the US Food and Drug Administration, FDA). Ciprofloxacin, like other fluoroquinolones, prolongs the QT interval; ventricular arrhythmias and torsades de pointes have been reported with it, mostly in patients with risk factors (ciprofloxacin SmPC). The effects add up; the risk is higher with congenital long QT, low potassium or magnesium, heart disease, bradycardia, in older people and in women (ciprofloxacin and sertraline SmPCs).

Symptoms

QT prolongation on ECG. Dizziness, syncope, palpitations. Rarely: polymorphic ventricular tachycardia (torsades de pointes). Risk is higher with hypokalaemia, hypomagnesaemia, bradycardia, and ischaemic heart disease.

Management

The US Food and Drug Administration (FDA) label of sertraline says to avoid drugs known to prolong the QT interval; its examples include antibiotics, among them fluoroquinolones (sertraline FDA label). European SmPCs are softer and allow the combination with caution: sertraline and ciprofloxacin are used carefully in patients with risk factors for QT prolongation, such as congenital long QT, low potassium or magnesium, heart disease or bradycardia, and ciprofloxacin also in older people and women (sertraline and ciprofloxacin SmPCs). Avoid the combination where possible; if ciprofloxacin is still needed, assess these risk factors before starting.

Mechanism

Ciprofloxacin inhibits hepatic CYP1A2, the enzyme that clears theophylline, so theophylline levels rise. The FDA theophylline label puts the increase at about 40%. Theophylline has a narrow margin: per the same label, above 20 mcg/mL both the frequency and severity of adverse reactions increase. The FDA ciprofloxacin label reports serious and fatal reactions with this combination, including cardiac arrest, seizures, status epilepticus and respiratory failure.

Symptoms

Tachycardia, tremor, nausea, vomiting, seizures, arrhythmias when theophylline above 20 mcg/mL.

Management

The combination is avoided, as the FDA ciprofloxacin label states, though no label lists it as a contraindication. A patient on theophylline gets an antibiotic without this interaction, for example amoxicillin-clavulanate for pneumonia. Levofloxacin does not change theophylline levels according to its SmPC, but together with theophylline it markedly lowers the seizure threshold. If ciprofloxacin is still needed, the theophylline SmPC caps the theophylline dose at no more than 60% of the recommended dose. Serum theophylline is measured during the course and after ciprofloxacin stops, and the dose follows the result. Nausea, vomiting, tremor or palpitations call for a same-day level.

Mechanism

Dual risk: both drugs lower the seizure threshold. Ciprofloxacin also blocks CYP1A2 (a minor tramadol activation route) and weakly affects serotonergic activity.

Symptoms

Seizures, especially in patients with prior epilepsy or head injury. Serotonin syndrome may occur with other serotonergics on board (agitation, tremor, hyperthermia).

Management

Avoid the combination in patients with prior epilepsy. Alternative antibiotics: a cephalosporin or nitrofurantoin (no seizure threshold effect). Alternative analgesics: paracetamol, an NSAID, or morphine.

Mechanism

Old experimental data suggested NSAIDs combined with fluoroquinolones increase CNS excitability and seizure risk (via GABAergic suppression). Clinical data are very limited.

Symptoms

Mostly asymptomatic. In patients with prior epilepsy or head injury: rare seizure cases.

Management

The combination is acceptable in patients without prior epilepsy. In epilepsy or eating disorders: alternative antibiotics (a cephalosporin, nitrofurantoin) and paracetamol for analgesia.

Mechanism

Amlodipine is broken down in the liver by the CYP3A4 enzyme. The US Food and Drug Administration (FDA) classifies ciprofloxacin as a moderate CYP3A4 inhibitor (FDA table of CYP inhibitors). Moderate and strong inhibitors of this enzyme raise amlodipine blood levels and with them the risk of low blood pressure, more so in older patients (amlodipine SmPC; amlodipine FDA label). The labels give no specific data for this pair.

Symptoms

Dizziness and weakness, especially on standing, from low blood pressure; swelling of the ankles and feet (amlodipine labels). These effects are more marked in older patients (amlodipine SmPC).

Management

During the ciprofloxacin course, watch for signs of low blood pressure and for swelling to decide whether the amlodipine dose needs adjusting (amlodipine label of the US Food and Drug Administration, FDA). The Spanish amlodipine SmPC also states that moderate CYP3A4 inhibitors may require clinical monitoring and a dose adjustment, especially in older patients (amlodipine SmPC). Raise the amlodipine dose with caution in older patients (amlodipine SmPC).

Mechanism

Ciprofloxacin weakly affects CYP3A4 and P-glycoprotein — the main clearance routes of apixaban. It also suppresses vitamin K-producing gut flora. Additive bleeding risk.

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 ciprofloxacin courses (5–7 days), do not change the apixaban dose but monitor for bleeding signs. In patients over 75 or with creatinine clearance below 50 mL/min, choose an alternative antibiotic without P-glycoprotein effect — ceftriaxone, cefixime, amoxicillin-clavulanate.

Mechanism

Ciprofloxacin is a weak CYP3A4 inhibitor. Atorvastatin plasma levels may rise slightly during short ciprofloxacin courses.

Symptoms

Usually no changes. In patients with myopathy risk factors: rare muscle pain.

Management

For short ciprofloxacin courses (5–7 days), no special adjustment. In patients with myopathy risk factors, check creatine kinase on muscle symptoms.

Mechanism

Calcium forms insoluble chelates with ciprofloxacin in the gut. Antibiotic bioavailability falls by 40–50%, raising the risk of clinical failure.

Symptoms

Persistent fever, infection symptoms, and inflammation markers 48–72 hours after antibiotic start.

Management

Take ciprofloxacin 2 hours before or 6 hours after calcium carbonate. Apply the same interval for iron, zinc, magnesium, and aluminium-containing antacids. If timing separation is impossible (e.g. continuous calcium need), choose an alternative antibiotic (amoxicillin-clavulanate, a cephalosporin).

Mechanism

In 10–15% of patients, gut flora (mainly Eubacterium lentum) metabolises part of digoxin to inactive products. Ciprofloxacin suppresses this flora — digoxin levels rise by 30–80% in those patients, increasing toxicity risk.

Symptoms

Nausea, vomiting, loss of appetite, confusion, yellow-green halos around lights, arrhythmia (bigeminy, AV block). Symptoms appear earlier in older patients and chronic kidney disease.

Management

For short ciprofloxacin courses (5–7 days), most patients need no adjustment. In older patients and chronic kidney disease, check digoxin level at 5–7 days after antibiotic start. With toxicity signs, reduce dose by 25%. Alternative antibiotics: a cephalosporin (minimal gut flora impact).

Mechanism

Fluoroquinolones disturb glucose homeostasis — they can cause both hypoglycaemia (especially in older patients on glucose-lowering therapy) and hyperglycaemia. The mechanism involves pancreatic beta-cell effects.

Symptoms

Hypoglycaemia: sweating, tremor, hunger, confusion at glucose below 3.9 mmol/L. Hyperglycaemia: thirst, polyuria, fatigue.

Management

For short ciprofloxacin courses (5–7 days) on metformin, intensify glucose self-monitoring (twice daily) for the first week. In older patients and diabetics on insulin or sulfonylureas, choose an alternative antibiotic (cephalosporin, amoxicillin-clavulanate).

Mechanism

Ciprofloxacin absorbs better in acidic gastric environment. Omeprazole raises gastric pH — ciprofloxacin bioavailability drops by 20–30%. Clinically significant in infections with borderline pathogen sensitivity.

Symptoms

In severe infection: inadequate clinical response — persistent fever, symptoms, and inflammation markers 48–72 hours after antibiotic start.

Management

On the combination, take ciprofloxacin 2 hours before or 6 hours after omeprazole. If dosing cannot be separated, alternative antibiotics: amoxicillin-clavulanate for respiratory infections, a cephalosporin for UTIs. In urinary infections, ciprofloxacin still reaches adequate urine concentrations regardless of bioavailability.

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

NIH ODS lists quinolone antibiotics among the drugs zinc interacts with, through the same chelation mechanism as the other divalent cations. The described magnitude is smaller than with calcium or iron, though the direction is the same both ways: the quinolone drops and so does zinc.

Symptoms

No symptoms of its own. It shows as reduced antibiotic efficacy over longer courses.

Management

During the course, keep zinc away from the antibiotic on the same basis as the other minerals. For week-long courses many patients prefer to pause zinc and resume afterwards, which is reasonable unless there is a specific indication.

Mechanism

The historical theory of reduced combined oral contraceptive efficacy via gut flora suppression is rejected by meta-analyses. Ciprofloxacin does not induce CYP3A4 and does not affect ethinylestradiol metabolism.

Symptoms

The combination usually causes no specific symptoms. Each drug's individual side effects remain.

Management

Additional contraception during short ciprofloxacin courses is not needed. The old "use a barrier method for the whole cycle" advice is outdated.