Warfarin

Explanation

Sulphonamides, such as sulphadoxine, have been associated with increased warfarin effect through multiple mechanisms: CYP2C9 inhibition (metabolism of active S-warfarin), displacement of protein binding and, potentially, reduction of the gut flora producing vitamin K. Pyrimethamine contributes less, but the two are used together in malaria prophylaxis and treatment. The INR should be monitored at the start and end of treatment and the warfarin dose adjusted; watch for bleeding signs.

Explanation

Theophylline may partially compete for CYP1A2/CYP2C9 metabolism of warfarin, slightly increasing INR. The effect is generally modest but clinically relevant in patients with INR already at the upper limit. Monitor INR during the first 2-3 weeks.

Explanation

Systemic corticosteroids may induce CYP3A4, accelerating warfarin metabolism and reducing INR. The effect is dose-dependent and more pronounced at high doses (>20 mg/day prednisolone) and prolonged treatments (>2 weeks). Monitor INR during and after course.

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Explanation

The warfarin label identifies antibiotics, including penicillins, as drugs that can alter INR in anticoagulated patients. The mechanism is multiple: reduction of vitamin K-producing gut flora, metabolic inhibition, and the infectious disease itself altering the coagulation state. With benzathine benzylpenicillin (long-acting IM injection, e.g., rheumatic fever prophylaxis), the effect on INR may be less predictable and more delayed. Monitor INR during and after the antibiotic course, watch for bleeding signs and adjust the warfarin dose according to the values.

Explanation

Cimetidine is a non-selective CYP450 inhibitor (including CYP2C9, the main enzyme metabolising active S-warfarin), so it reduces warfarin clearance and raises INR in a clinically relevant way. QUADRO 2 of Annex 7 lists cimetidine among the drugs that increase the effect of oral anticoagulants. The effect is dose-dependent and usually appears within the first days of starting cimetidine. Patients with previously stable INR are the most exposed to bleeding, especially the elderly and those on multiple drugs. If an H2 blocker is needed, prefer famotidine or a proton pump inhibitor (no relevant CYP2C9 interaction); if cimetidine is kept, intensify INR monitoring and adjust the warfarin dose.

Explanation

Soybean oil-based lipid emulsions (such as Intralipid) contain vitamin K, the natural antidote to warfarin: dietary/intravenous vitamin K reactivates the vitamin K-dependent clotting factors and reduces the anticoagulant effect, lowering INR. In anticoagulated patients receiving parenteral nutrition with lipid emulsion, the vitamin K load can make INR lower and more variable, increasing thrombotic risk if not compensated. Monitor INR more frequently after starting (or changing) the lipid emulsion and adjust the warfarin dose according to the values.

Explanation

Thyroid status directly influences the turnover of vitamin K-dependent clotting factors: in hypothyroidism factor degradation is slower, intensifying warfarin effect; in hyperthyroidism the opposite occurs. QUADRO 2 of Annex 7 records this interaction in the oral anticoagulants section. In practice, when starting or adjusting levothyroxine, the warfarin response may vary over weeks, tracking the normalisation of TSH levels. The risk is greatest in patients with stabilised warfarin doses who start thyroid replacement. Monitor INR weekly during levothyroxine adjustment and after reaching euthyroidism; warn the patient of bleeding or thrombotic signs.

Explanation

The tamoxifen SmPC (EMC-UK) reports a significant increase in anticoagulant effect when combined with coumarins, and the DailyMed label contraindicates use in patients requiring concomitant coumarin anticoagulation (in the risk-reduction indication). The mechanism involves inhibition of coumarin metabolism and an additive effect on haemostasis. In adjuvant breast cancer therapy, if anticoagulation is essential, monitor INR closely after starting tamoxifen (the anticoagulant dose may need to be reduced).

Explanation

The approved methimazole label (DailyMed) states that, through potential inhibition of vitamin K activity, the drug may increase the activity of oral anticoagulants (e.g. warfarin), recommending additional PT/INR monitoring, especially before surgical procedures. The effect is more relevant as hyperthyroidism is controlled, when drug clearance changes.

Explanation

The gliclazide SmPC (EMC-UK) states that sulfonylureas may potentiate anticoagulation during concurrent treatment with coumarins (e.g. warfarin), and the anticoagulant dose may need adjustment. The mechanism involves displacement from protein binding and inhibition of coumarin metabolism. The combination is common (type 2 diabetes and atrial fibrillation).

Explanation

Amiodarone is a potent inhibitor of CYP2C9, the isoenzyme that metabolises S-warfarin (the more active enantiomer), and also inhibits CYP2C19 and CYP3A4. The result is a marked rise in the INR, which can start within the first days but continues to evolve for weeks because amiodarone has a very long half-life (20–100 days) and accumulates in tissues. The approved label recommends reducing the warfarin dose by 30–50% when amiodarone is started and monitoring the INR closely; the effect persists for weeks to months after amiodarone is stopped, so warfarin must be re-adjusted during that phase. This combination is common in atrial fibrillation patients and demands disciplined INR control and vigilance for bleeding.

Explanation

Ampicillin, like other broad-spectrum antibiotics, reduces the gut bacterial flora that synthesises vitamin K. Since warfarin inhibits vitamin K epoxide reductase, the lower availability of endogenous vitamin K translates into a higher INR and bleeding risk. The interaction is more relevant in patients with marginal nutrition, liver disease or prolonged antibiotic therapy. INR should be monitored during treatment and after its completion, with vigilance for bleeding signs; adjust the warfarin dose according to the INR.

Explanation

Atazanavir inhibits CYP3A4 (and UGT1A1), enzymes involved in the metabolism of both warfarin enantiomers. The inhibition can increase warfarin exposure and raise the INR, with increased bleeding risk. Since atazanavir is used with ritonavir or cobicistat (also inhibitors), the effect can be even more pronounced. The INR should be monitored frequently at initiation, during and after antiretroviral discontinuation, and the warfarin dose adjusted accordingly.

Explanation

Atovaquone is very highly protein-bound (over 99%) and, by competing for the same binding sites, can displace warfarin and increase the active free fraction, with a transient INR rise. Proguanil does not appear to contribute significantly, but the two are used together. The effect is usually modest and transient, but in patients with a borderline INR it can trigger bleeding. The INR should be monitored at the start and end of prophylaxis and the dose adjusted if necessary.

Explanation

Azithromycin, like other macrolides, can increase the effect of warfarin through two mechanisms: reduction of the gut flora that produces vitamin K and inhibition (milder than clarithromycin and erythromycin) of CYP3A4, which participates in warfarin metabolism. Clinical reports with azithromycin are less consistent, but cases of elevated INR and bleeding exist. The warfarin label lists macrolides as drugs that can increase the anticoagulant effect. The INR should be monitored during the antibiotic course and in the following weeks, with dose adjustment if needed.

Explanation

Ceftriaxone is a broad-spectrum cephalosporin that reduces the gut flora producing vitamin K. Since warfarin is a vitamin K antagonist, the lower endogenous availability of the cofactor raises the INR and the bleeding risk. The effect is more relevant in malnourished, elderly, liver-disease or prolonged-antibiotic patients. Monitor the INR during and after treatment and adjust the warfarin dose; watch for bleeding signs.

Explanation

Ketoconazole is a potent inhibitor of several cytochrome P450 isoenzymes, including CYP2C9 (which metabolises active S-warfarin) and CYP3A4 (which metabolises R-warfarin). The inhibition results in increased warfarin levels, INR elevation and significant bleeding risk — the ketoconazole and warfarin labels both mention this interaction. Whenever possible the combination should be avoided (alternative antifungals with fewer interactions exist); if unavoidable, reduce the warfarin dose, monitor the INR frequently at initiation and watch for bleeding signs.

Explanation

Ciprofloxacin increases the effect of warfarin through three pathways: inhibition of CYP1A2 and CYP3A4 (enzymes that metabolise warfarin), reduction of the gut flora producing vitamin K and, potentially, displacement of protein binding. There are reports of marked INR elevations and severe, sometimes fatal, bleeding, so the warfarin label recommends INR monitoring when a fluoroquinolone is started or stopped. The interaction is more pronounced in the elderly and in patients with multiple comorbidities. Monitor the INR frequently during and after the antibiotic course and adjust the dose.

Explanation

Clarithromycin is a potent CYP3A4 inhibitor, the main enzyme in R-warfarin metabolism, and simultaneously reduces the gut flora producing vitamin K. The result is INR elevation with increased bleeding risk, documented in many reports and mentioned in approved labels. The effect can appear within days of starting and persist after the antibiotic is stopped. Whenever possible, choose an alternative antibiotic (e.g. azithromycin has lower interaction potential); if clarithromycin is needed, monitor the INR frequently and reduce the warfarin dose as needed.

Explanation

Chloroquine and its analogues can inhibit warfarin metabolism and raise the INR, although the exact mechanism is not fully established (possible CYP2C9 inhibition and an effect on gut flora). Reports exist of INR elevation and bleeding when chloroquine is added to warfarin. The INR should be monitored at the start and end of antimalarial treatment and the warfarin dose adjusted; watch for bleeding signs, especially in prolonged treatment (e.g. systemic lupus erythematosus, rheumatoid arthritis).

Explanation

Efavirenz induces CYP3A4 and CYP2C9 (among other isoenzymes), accelerating warfarin metabolism and reducing its plasma levels. The usual result is a decrease in the INR, with thrombosis risk if the dose is not adjusted; however, the effect varies between patients and there may be phases of INR instability in both directions. The INR should be monitored frequently at efavirenz initiation and discontinuation and the warfarin dose adjusted accordingly, with vigilance for thromboembolism and bleeding signs.

Explanation

Erythromycin inhibits CYP3A4, the enzyme that metabolises R-warfarin, and reduces the gut flora producing vitamin K. This dual action increases the anticoagulant effect, with INR elevation and bleeding risk documented in clinical reports and mentioned in approved labels. The effect can be rapid (days) and persist after the antibiotic ends. Monitor the INR frequently during and after treatment and adjust the warfarin dose; consider an alternative antibiotic when possible.

Explanation

Fluconazole is a potent inhibitor of CYP2C9, the isoenzyme that metabolises active S-warfarin; the effect is dose-dependent and more marked at high doses (e.g. 400 mg/day). The inhibition raises warfarin levels and the INR, with significant bleeding risk — a well-documented interaction mentioned in the approved fluconazole and warfarin labels. Reduce the warfarin dose when starting fluconazole, monitor the INR frequently and re-adjust after the antifungal is stopped; watch for bleeding signs.

Explanation

Hydroxychloroquine, like chloroquine, can inhibit warfarin metabolism and raise the INR, although the mechanism is not fully established. Reports are more frequent in patients with systemic lupus erythematosus or rheumatoid arthritis on chronic treatment, in whom the interaction can manifest weeks after initiation. The INR should be monitored at initiation and discontinuation of hydroxychloroquine and the warfarin dose adjusted; watch for bleeding signs.

Explanation

Isoniazid inhibits several cytochrome P450 isoenzymes, including CYP2C9 and CYP3A4, and can reduce warfarin metabolism, raising the INR. The effect is variable and coexists with the complexity of antituberculosis regimens (which often include rifampicin, a potent inducer — the interaction network is complex and the INR can swing in both directions). The INR should be monitored frequently when starting, adjusting and stopping each component of the antituberculosis regimen, and the warfarin dose adjusted accordingly.

Explanation

Itraconazole is a potent CYP3A4 inhibitor and also affects CYP2C9, enzymes involved in warfarin metabolism. The inhibition results in increased warfarin levels, INR elevation and bleeding risk — an interaction mentioned in approved labels. Whenever possible, choose an alternative antifungal with lower interaction potential; if itraconazole is unavoidable, reduce the warfarin dose, monitor the INR frequently at initiation and during treatment and watch for bleeding signs.

Explanation

Levofloxacin, like other fluoroquinolones, can potentiate warfarin by reducing the gut flora producing vitamin K and by inhibiting cytochrome P450 isoenzymes (CYP1A2 and CYP3A4), although its inhibition potential is lower than ciprofloxacin's. Reports exist of INR elevation and bleeding. The INR should be monitored during and after the antibiotic course and the warfarin dose adjusted; the risk is higher in the elderly and in patients with reduced hepatic or renal function.

Explanation

Metronidazole is a potent inhibitor of CYP2C9, the enzyme that metabolises active S-warfarin, and has been associated with marked INR elevations and bleeding, including severe cases. The effect can appear within days of starting the antibiotic. The warfarin label lists metronidazole among the drugs that potentiate its effect. The INR should be monitored frequently during and after treatment, a warfarin dose reduction should be considered, and bleeding signs watched for; in patients with a borderline INR the interaction can be particularly dangerous.

Explanation

Nevirapine induces CYP3A4 and CYP2B6 and can accelerate warfarin metabolism, reducing its levels and the INR. The risk is loss of anticoagulation and thromboembolism if the dose is not adjusted; the inducing effect develops over 2–4 weeks and persists after discontinuation. The INR should be monitored frequently at nevirapine initiation and discontinuation and the warfarin dose adjusted accordingly, with vigilance for thrombosis signs.

Explanation

Piperacillin-tazobactam is a broad-spectrum antibiotic that reduces the gut flora producing vitamin K, decreasing the availability of the cofactor for carboxylation of the vitamin K-dependent coagulation factors. Since warfarin acts by inhibiting vitamin K epoxide reductase, the anticoagulant effect is potentiated and the INR rises. The risk is higher in critically ill, malnourished, liver-disease patients or with prolonged treatment — a frequent context for this antibiotic. Monitor the INR during and after treatment and adjust the warfarin dose; watch for bleeding.

Explanation

Quinine inhibits warfarin metabolism (CYP2C9 and CYP3A4) and, through its high plasma protein binding, can displace the active free fraction. The result is a marked INR elevation with bleeding risk — a well-documented interaction, with reports of severe bleeding in patients using quinine (or quinine-containing tonic water in large amounts) with warfarin. The INR should be monitored frequently during treatment and after discontinuation, the warfarin dose reduced if needed, and the patient alerted to bleeding signs.

Explanation

Rifampicin is one of the most potent inducers of CYP2C9 and CYP3A4, the enzymes that metabolise warfarin. The induction accelerates warfarin elimination and markedly reduces the INR, with thromboembolism risk; studies show reductions of more than 50% in warfarin exposure. Some patients need the warfarin dose doubled or even tripled, with frequent INR monitoring, and the dose must be re-adjusted in the 1–2 weeks after rifampicin is stopped, because the inducing effect fades gradually and the INR can then rise. This interaction is particularly relevant in tuberculosis treatment in anticoagulated patients.

Explanation

Ritonavir has a complex pharmacokinetic profile: it is a potent inhibitor of CYP3A4 and CYP2D6, but also an inducer of other enzymes and transporters. The net effect on warfarin is variable and unpredictable between patients — most reports show an INR increase, but cases of reduction exist. When ritonavir is used at low boosting doses (with other antiretrovirals), the effect is equally variable. The INR should be monitored frequently at initiation, during and after ritonavir discontinuation, adjusting the warfarin dose according to the result and alerting for bleeding and thrombosis signs.

Explanation

Voriconazole inhibits CYP2C9 (which metabolises active S-warfarin) and CYP3A4, with a clinically relevant documented effect: studies at therapeutic doses showed substantial increases in the INR and warfarin exposure when voriconazole is added. The voriconazole label recommends INR monitoring and warfarin dose adjustment. The interaction is particularly relevant in patients with invasive aspergillosis, often already in a fragile clinical state. Monitor the INR frequently at voriconazole initiation and discontinuation and watch for bleeding signs.

Explanation

Colchicine is a substrate and inhibitor of CYP3A4, an enzyme involved in warfarin metabolism; the inhibition can reduce warfarin clearance and raise the INR. The interaction is especially relevant in prolonged treatment (e.g. gout prophylaxis, familial Mediterranean fever), in which the effect accumulates. Monitor the INR when starting colchicine and after dose changes, and watch for bleeding signs; in renal or hepatic impairment, the toxicity risk of both drugs increases.

Explanation

The combination of warfarin and dabigatran (two anticoagulants with different mechanisms — vitamin K antagonism and direct thrombin inhibition) has no therapeutic indication and multiplies the risk of major bleeding, including intracranial haemorrhage. Switching between the two drugs must follow a protocol: when changing from warfarin to dabigatran, stop warfarin and start dabigatran when the INR is below the threshold; conversely, start warfarin a few days before stopping dabigatran, monitoring the INR. Dual anticoagulation can occur transiently in poorly managed switches or by medication error — alert the patient to bleeding signs.

Explanation

Corticosteroids, including dexamethasone, can modify the response to warfarin in both directions: most reports show an INR decrease (possibly through enzyme induction and fluid retention), but cases of increased effect exist, and the response is unpredictable between patients. The interaction is most relevant in prolonged courses or high doses. The INR should be monitored frequently when starting, adjusting and stopping dexamethasone and the warfarin dose adjusted accordingly; the INR must be re-evaluated after corticosteroid discontinuation.

Explanation

Diclofenac, like other NSAIDs, increases the bleeding risk in warfarin patients through three pathways: reversible platelet aggregation inhibition, gastric mucosal injury (ulcer and GI bleeding risk) and possible interference with warfarin pharmacokinetics. The risk of serious gastrointestinal bleeding with the NSAID + anticoagulant combination is substantial and well documented. Whenever possible, use alternative analgesics (paracetamol at controlled doses); if diclofenac is unavoidable, use the lowest effective dose for the shortest time, consider gastroprotection (PPI) and monitor the INR and bleeding signs.

Explanation

Epoetin alfa stimulates erythropoiesis and can alter the response to warfarin: the accelerated production of red cells increases vitamin K consumption (needed for synthesis of coagulation factors) and can lower the INR, sometimes requiring a warfarin dose increase. Reports also exist of variation in both directions. The interaction is most relevant in chronic kidney disease patients on prolonged epoetin treatment. The INR should be monitored frequently when starting and adjusting epoetin and during treatment, adjusting the warfarin dose as needed.

Explanation

The phenytoin-warfarin interaction is one of the most complex in clinical pharmacology: phenytoin induces CYP2C9 and CYP2C19 (which can reduce warfarin's effect) but also competes with warfarin for plasma protein binding, and warfarin itself inhibits phenytoin metabolism, raising its levels and the risk of neurological toxicity (nystagmus, ataxia, sedation). The net effect on the INR is unpredictable and can oscillate in the first weeks. The INR and serum phenytoin levels should be monitored at initiation and whenever either drug is adjusted, with gradual dose changes.

Explanation

Phenobarbital is a potent inducer of CYP2C9, CYP2C19 and CYP3A4, the enzymes that metabolise warfarin, and accelerates its elimination — the anticoagulant effect decreases and the INR falls, with thromboembolism risk if the dose is not adjusted. The inducing effect takes 1–3 weeks to fully develop and persists for several weeks after phenobarbital is stopped, when the INR can rise sharply. The INR should be monitored frequently when starting, during and after phenobarbital, increasing the warfarin dose as needed and re-adjusting it during the withdrawal phase.

Explanation

Phytomenadione (vitamin K1) is the antidote to warfarin: it replenishes the cofactor needed to carboxylate factors II, VII, IX and X and reverses the anticoagulant effect, lowering the INR within hours. This interaction is intentional in the reversal of supratherapeutic INR or bleeding, but accidental (e.g. multivitamin supplements, vitamin-K-rich diet) causes INR instability and thrombosis risk. After vitamin K administration, warfarin may take days to weeks to return to therapeutic effect, and the dose must be re-adjusted with INR monitoring. In anticoagulated patients, advise consistency in vitamin K intake and review supplements.

Explanation

Leflunomide (and its active metabolite teriflunomide) has been associated with INR elevation in warfarin-anticoagulated patients, with reports of severe bleeding. The mechanism is not fully established but probably involves interference with warfarin metabolism. The effect can appear weeks after starting leflunomide. The INR should be monitored frequently when starting leflunomide and after dose changes, and bleeding signs watched for; if rapid discontinuation is needed, consider the cholestyramine washout procedure described in the label.

Explanation

Metamizole (dipyrone) can increase the effect of warfarin through a pharmacokinetic interaction (metabolism inhibition, with INR elevation) and its own antiplatelet effect; cases of severe bleeding, including gastrointestinal bleeding, have been described. The EMA, in its metamizole review, mentions the need for INR monitoring in anticoagulated patients. Whenever possible, use alternative analgesics (paracetamol at controlled doses); if metamizole is used, monitor the INR frequently and alert for bleeding signs.

Explanation

Naproxen, like other NSAIDs, increases the bleeding risk in warfarin patients: it inhibits platelet aggregation (prolongedly, given its long half-life), injures the gastric mucosa and can interfere with warfarin pharmacokinetics. The NSAID + anticoagulant combination is associated with a substantial risk of serious gastrointestinal bleeding. Whenever possible, use alternative analgesics; if naproxen is unavoidable, use the lowest effective dose for the shortest time, consider gastroprotection and monitor the INR and bleeding signs.

Explanation

Omeprazole inhibits CYP2C19, an enzyme involved in R-warfarin metabolism; the inhibition can reduce warfarin clearance and raise the INR, although the effect is generally modest and variable between patients. Other proton pump inhibitors with less CYP2C19 affinity (e.g. pantoprazole) are sometimes preferred in anticoagulated patients. The INR should be monitored when omeprazole is started or stopped, especially in patients with a borderline INR or relevant genetic polymorphisms, and the dose adjusted as needed.

Explanation

Orlistat inhibits gastrointestinal lipases and reduces absorption of dietary fats and fat-soluble vitamins, including vitamin K. Since warfarin is a vitamin K antagonist, the lower availability of the cofactor can increase the anticoagulant effect and the INR; cases of elevated INR and bleeding have been described, especially in the first weeks of treatment. The INR should be monitored at initiation and during orlistat treatment and the warfarin dose adjusted; maintain consistent vitamin K intake and watch for bleeding signs.

Explanation

Paracetamol is the preferred analgesic in anticoagulated patients because it lacks the gastrointestinal bleeding risk of NSAIDs, but at high doses or with prolonged use it can raise the INR: the mechanism involves glutathione depletion and interference with the synthesis of vitamin K-dependent coagulation factors (the NAPQI metabolite inhibits vitamin K-dependent carboxylation). Studies show INR elevations with doses ≥ 2 g/day maintained for several days. Use the lowest effective dose, do not exceed 2–3 g/day continuously and monitor the INR in patients using paracetamol chronically.

Explanation

The combination of warfarin and rivaroxaban (vitamin K antagonist + direct factor Xa inhibitor) has no therapeutic indication and substantially increases the risk of major bleeding, including intracranial haemorrhage. Switching between the two drugs must follow a protocol: when changing from warfarin to rivaroxaban, stop warfarin and start rivaroxaban when the INR is below the threshold (usually < 3.0); conversely, start warfarin overlapping with rivaroxaban until the INR is in the therapeutic range, then stop rivaroxaban. Prolonged dual anticoagulation is a medication error — alert the patient to bleeding signs.

Explanation

Valproate is extensively protein-bound (90% or more) and can displace warfarin from its binding sites, increasing the active free fraction and the anticoagulant effect; it can also interfere with warfarin metabolism. Reports exist of INR elevation and bleeding when valproate is added to warfarin. The INR should be monitored at initiation and during treatment and the warfarin dose adjusted; the effect is more relevant in patients with hypoalbuminaemia (elderly, liver disease), in whom the free fraction of both drugs is higher.

Explanation

Tetracyclines have been shown to depress plasma prothrombin activity, potentiating the effect of oral anticoagulants such as warfarin. In patients on anticoagulant therapy starting a tetracycline, a downward adjustment of the anticoagulant dose may be required. The INR should be closely monitored at the start and end of the antibiotic, and the patient alerted to signs of bleeding — gum bleeding, bruising or dark stools. The interaction is especially relevant in the elderly and in patients with unstable INR.

Explanation

Like other tetracyclines, minocycline may depress plasma prothrombin activity and potentiate the effect of oral anticoagulants. In patients on warfarin starting minocycline, the INR should be monitored at the start and end of the antibiotic, with adjustment of the anticoagulant dose as needed. Alert the patient to signs of bleeding (gum bleeding, bruising, dark stools), especially in the elderly or those with unstable INR.

Explanation

The flutamide label documents increases in prothrombin time in patients receiving warfarin ("Increases in prothrombin time have been noted in patients receiving warfarin therapy"), and the Prontuário records the same ("Increases the effect of warfarin"). The mechanism is not fully established, but the effect translates into increased bleeding risk. The prostate cancer patient on combined therapy with warfarin (e.g., for atrial fibrillation or thromboembolism) should have frequent INR in the first weeks of the combination and after any dose change of either drug. Adjust the warfarin dose to target and instruct the patient to watch for bleeding signs (gums, epistaxis, bruising, dark stools).

Explanation

The megestrol label is explicit: "Megestrol acetate may interact with warfarin and increase International Normalized Ratio (INR). Closely monitor INR in patients taking megestrol acetate and warfarin". The mechanism is not fully established, but the effect on INR may be clinically relevant, especially in oncology patients with cachexia whose nutritional status is already unstable. When starting, adjusting or stopping megestrol, monitor INR more frequently and adjust the warfarin dose to target. Instruct the patient to report bleeding signs (gums, epistaxis, bruising, dark stools) and to avoid NSAIDs or other anticoagulants without supervision.

Explanation

The FDA fondaparinux label documents the increased bleeding risk when combined with drugs affecting haemostasis (including warfarin, NSAIDs and antiplatelets). Fondaparinux is a selective factor Xa inhibitor and warfarin antagonises the synthesis of vitamin K-dependent factors: the mechanisms are complementary, so the risk of major bleeding (including intracranial and retroperitoneal) is additive. Overlap is common in the therapeutic bridge to warfarin (e.g., DVT/PE), but requires active vigilance: monitor for signs of bleeding, haemoglobin and, during prolonged overlap, INR until the therapeutic range is reached. Higher-risk patients are the elderly, low body weight and renal impairment, which accumulate fondaparinux. There is no absolute contraindication — the combination is standardised in protocols — but dual anticoagulation should be limited to the bridge period and monitored.

Explanation

The Prontuário Terapêutico documents in the memantine monograph the possibility of potentiating the anticoagulant effect of warfarin. The mechanism is not fully clarified, but the interaction is clinically relevant because dementia patients (memantine indication) are frequently elderly and polymedicated, including anticoagulated. The risk is warfarin over-anticoagulation with supratherapeutic INR and bleeding (bruising, gum bleeding, melaena) when memantine is started or adjusted. The practical guidance is to monitor INR after starting, adjusting or stopping memantine in patients anticoagulated with warfarin, and watch for bleeding signs; the same precaution applies to the new oral anticoagulants, although less documented.

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Explanation

Celecoxib is metabolised by CYP2C9 and moderately inhibits this enzyme (Ki ~4 μM). S-warfarin (more potent enantiomer, 3-5x more active than R-warfarin) is metabolised almost exclusively by CYP2C9. The resulting inhibition increases S-warfarin levels 1.5-3-fold and INR by 1.5-3 units. The effect is more pronounced in CYP2C9 poor metabolisers (CYP2C9*3 — frequency ~7% in Caucasians). FDA recommends reducing warfarin dose by 25-50% and monitoring INR 2-3 times/week during the first 2 weeks.

Explanation

SSRIs inhibit platelet serotonin reuptake, reducing platelet aggregation. Combined with warfarin (anticoagulant), bleeding risk increases moderately (OR 1.5-2x). The mechanism is additive (platelet + coagulation). Monitor INR and signs of bleeding. Fluoxetine may also inhibit CYP2C9, slightly increasing S-warfarin levels. Maintain low warfarin dose if possible.

Explanation

Oxcarbazepine induces CYP3A4 and CYP2C9, metabolic pathways for warfarin (S and R enantiomers). Co-administration may significantly reduce INR, increasing thromboembolic risk. Monitor INR weekly during the first 4 weeks after initiation or dose change.

Explanation

High-dose vitamin C (several grams per day) has been associated with changes in INR in patients anticoagulated with warfarin — generally a reduced anticoagulant effect, although reports vary. The mechanism is not fully clarified (possible interference with warfarin absorption or metabolism). With usual doses (up to ~500 mg/day) the risk is minimal, but in patients taking megadoses of vitamin C, monitor INR after starting, changing or stopping the supplement, and adjust the warfarin dose as needed.

Explanation

Amoxicillin-clavulanate eliminates much of the gut bacterial flora that synthesises vitamin K, reducing the availability of the cofactor needed to carboxylate the vitamin K-dependent coagulation factors (II, VII, IX, X). Since warfarin acts precisely through that pathway (inhibition of vitamin K epoxide reductase), the additional reduction of endogenous vitamin K potentiates the anticoagulant effect and the INR rises. The effect is more evident in patients with marginal dietary vitamin K intake and can appear days after starting the antibiotic. INR monitoring during and after the antibiotic course is essential; in patients with a previously stable INR, consider more frequent checks and alert for bleeding signs (gingival bleeding, bruising, melaena).