PHENOBARBITAL AND WARFARIN AND OTHER ORAL ANTICOAGULANTS

The effects of a large number of oral anticoagulants (acenocoumarol, dicoumarol, ethyl biscoumacetate, phenprocoumon and warfarin) are reduced by the barbiturates.
Reduced anticoagulant effects may occur within 2 to 4 days (maximum effect after about 3 weeks). Monitor the INR closely until stable and be aware that dose increases of 30 to 60% are likely to be needed. Also monitor if the barbiturate is stopped (note that the interaction may persist for up to 6 weeks after concurrent use is stopped). The benzodiazepines do not usually interact with anticoagulants, and may therefore be a suitable alternative in some circumstances.