Recent studies have indicated that viral infections, influenza vaccination, or drugs that increase interferon synthesis all decrease hepatic drug metabolism. We report a case in which influenza vaccination was temporally related to an increased anticoagulant effect of warfarin. A prospective study evaluating the effect of influenza vaccination on the prothrombin time of eight patients anticoagulated over the long term showed that there was prolongation of prothrombin time of 40%. In a second study, the effect of influenza vaccination on warfarin t1/2 was determined in healthy subjects. No significant effect on warfarin metabolism was observed after vaccination. We conclude that influenza vaccination is associated with increased anticoagulant response in some patients receiving anticoagulants over a long term. This effect appears to be related to some step in the coagulation pathway and not to decreased warfarin metabolism and a subsequent rise in serum concentration.

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The hypocholesterolemic cation resins, cholestyramine and colestipol, have variable effects on the absorption parameters of a number of lipophillic, anion drugs. Because of the unpredictable nature of this interaction, we have assessed in human volunteers the effect of these resins on the rate and total absorption of phenytoin. Our results indicate that these resins do not affect the absorption parameters of phenytoin and that special care does not have to be exercised when these agents are co-administered.