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Rifampin and warfarin: a drug interaction
Annals of Internal Medicine
|February 1, 1975
Summary
Rifampin significantly increases warfarin dosage requirements, necessitating up to 20 mg daily for therapeutic effect. This drug interaction, potentially due to enzyme induction, resolves gradually after rifampin discontinuation.
Area of Science:
- Pharmacology
- Drug Metabolism
- Drug Interactions
Background:
- The interaction between warfarin and rifampin is not well-established.
- Rifampin may increase the required dosage of warfarin in patients taking both medications concurrently.
Purpose of the Study:
- To investigate the clinical significance of the drug interaction between warfarin and rifampin.
- To quantify the change in warfarin requirements during and after rifampin co-administration.
Main Methods:
- A case study observing a patient requiring warfarin therapy.
- Monitoring warfarin dosage and therapeutic effect during concomitant administration with rifampin.
- Assessing changes in warfarin requirement after rifampin withdrawal.
Main Results:
- Concomitant administration of rifampin and warfarin necessitated a high warfarin maintenance dose (20 mg/day).
- Withdrawal of rifampin resulted in a 50% decrease in the patient's warfarin requirement.
- The interaction's effects peaked 5-7 days after rifampin initiation and cessation.
Conclusions:
- Rifampin significantly increases warfarin dosage requirements, likely through induction of microsomal enzymes and enhanced warfarin catabolism.
- This interaction is clinically significant and requires careful monitoring of warfarin dosage when co-administered with rifampin.
- The onset and offset of the interaction are gradual, lasting approximately 5-7 days after initiation or withdrawal of rifampin.
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