Case reports suggest that cranberry juice can increase the anticoagulant effect of warfarin. We investigated the effects of cranberry juice on R-S-warfarin, tizanidine, and midazolam; probes of CYP2C9, CYP1A2, and CYP3A4. Ten healthy volunteers took 200 ml cranberry juice or water t.i.d. for 10 days. On day 5, they ingested 10 mg racemic R-S-warfarin, 1 mg tizanidine, and 0.5 mg midazolam, with juice or water, followed by monitoring of drug concentrations and thromboplastin time. Cranberry juice did not increase the peak plasma concentration or area under concentration-time curve (AUC) of the probe drugs or their metabolites, but slightly decreased (7%; P=0.051) the AUC of S-warfarin. Cranberry juice did not change the anticoagulant effect of warfarin. Daily ingestion of cranberry juice does not inhibit the activities of CYP2C9, CYP1A2, or CYP3A4. A pharmacokinetic mechanism for the cranberry juice-warfarin interaction seems unlikely.