Phenprocoumon Cimetidine

In 3 studies, the AUC of single-dose acenocoumarol was increased by cimetidine, and the prothrombin time prolonged, " with the effect greatest for A-acenocoumarol. However, another study found no interaction. Data from one patient taking acenocoumarol and one taking phenindione showed that cimetidine increased their anticoagulant effects. In one study in patients stabilised on phenprocoumon, cimetidine 400 mg twice daily did not alter the pharmacokinetics of phenprocoumon nor its anticoagulant effect. ... 

Cimetidine Potentiation (144) not documented for phenprocoumon (145) Stereoselective inhibition of warfarin metabolism (146) Adjust dosage... 

Harenberg J, Zimmermann R, Staiger C, de Vries JX, Walter E, Weber E. Lack of effect of cimetidine on action of phenprocoumon. Eur J Chn Pharmacol 1982 23(4) 365-7. 

The anticoagulant effects of warfarin can be increased by cimeti-dine. The effect is generally minor to modest, although severe bleeding has been reported in a few cases. Acenocoumarol seems to interact similarly, but phenprocoumon appears not to be affected. In one patient the effects of phenindione were modestly increased by cimetidine. Famotidine, nizatidine, ranitidine and roxatidine normally do not appear to interact, although isolated cases of bleeding have been reported. 

Cimetidine also appears to interact with acenocoumarol, but not phenprocoumon. The other H2-receptor antagonists normally do not act as enzyme inhibitors. 

The interaction between warfarin and cimetidine is well documented, well established and potentially clinically important. Its effects are generally modest, but rarely, patients have shown a marked interaction. Because of this unpredictability, and to avoid bleeding, the response should be monitored well in every patient when cimetidine is first added, being alert for the need to reduce the warfarin dosage. The onset of the interaction appears rapid effects have been seen within days, and even as early as 24 hours. The effect of low non-prescription doses of cimetidine on warfarin do not appear to have been studied. Acenocoumarol is reported to in-teraet similarly, and there is one case of phenindione being affected. Expect other coumarins and indanediones to behave in the same way, with the possible exception of phenprocoumon, which was not affected in one study. 

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