Dangerous Drug Interaction: Warfarin-Sulfa Drugs
| warfarin (Coumadin) |
erythromycin/sulfisoxazole (Pediazole)
sulfamethizole (Thiosulfil Forte)
sulfamethoxazole (Gantanol)
sulfisoxazole (Gantrisin)
trimethoprim/sulfamethoxazole (Bactrim DS, Bactrim SS, Cotrim DS, Cotrim SS, Septra DS, Sulfatrim) |
| Brand names appear in parentheses above and are trademarks of their respective manufacturers/owners. |
Impact: Potential for increased effects of warfarin1
Mechanism of Interaction: Currently, the interaction with sulfa drugs is unknown; however, clinicians hypothesize that warfarin's activity is prolonged due a decreased production of vitamin K by intestinal flora affected by systemic antibiotic administration. Additionally, a hypoprothrombinemic effect may occur when S-warfarin is combined with sulfaethoxazole/trimethoprim.3
Alternatives to Patient Management: Avoid concomitant use of a sulfa drug with warfarin, particularly sulfamethoxazole-trimethoprim. Identify microbial pathogen prior to initiation of antibiotic therapy. Consider culture sensitivity screening as research indicates cautious use of any antibiotic with warfarin. If use of a sulfa drug is imperative, then reduce warfarin dose by 50% during antibiotic administration and for one week following completion of the antibiotic. If sulfamethoxazole-trimethoprim therapy is required, then monitor INR every other day for elevating trends.2
Monitoring/Precautions: INR should be monitored during co-administration of warfarin with a sulfa drug other than sulfamethoxazole-trimethoprim and warfarin dose adjustments made as necessary.1 Signs and symptoms of an active bleed should be monitored daily with particular attention to the appearance and patterns of bruises. Signs of an active bleed include: coughing up blood in the form of coffee grinds (hemoptysis), gingival bleeding, nose bleeds, cola- or tea-colored urine (hematuria), and black, tarry stools (hemoccult positive?).
References: Follow this link for a complete list of references.
The above information serves only as a guide for use by qualified medical practitioners in understanding, handling and avoiding frequent and potentially dangerous drug interactions that occur in long-term care. This presentation is not intended to instruct a practitioner how to treat any medical condition, nor is it intended to replace a practitioner's best clinical judgment. AMDA expressly disclaims responsibility and liability for any adverse effects, damages or other consequences resulting from the use of any of the information contained in this presentation.
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