Cimetidine INDEX

Theophylline is a narrow therapeutic index drug with significant difference in bioavailability following oral administration. The half-life of the drug is increased by heart failure, cirrhosis and viral infections, in elderly patients, and by certain drugs, such as cimetidine, ciprofloxacin, oral contraceptives and fluvoxamine. The half-life is decreased in smokers, chronic alcoholism, and by certain drugs, such as phenytoin, rifampicin and carbamazepine. 

Based on current information, inhibitors of CYP 1A2 have the greatest potential for causing interactions with olanzapine. Examples include cimetidine, fluvoxamine, and fluoroquinolone antibiotics (e.g., ciprofloxacin) to varying degrees. To date, however, no serious inhibition interactions have been reported with olanzapine, which may be a result of olanzapine s wide therapeutic index. Carbamazepine has been reported to increase olanzapine elimination by as much as 50%. Cigarette smoking is a potent inducer of CYP 1A2, and one would expect lower mean olanzapine serum concentrations in smokers compared to nonsmokers. 

Cimetidine (but not ranitidine) or rifampin reduces the plasmic level of tocainide, whereas metoprolol has added effects with tocainide on wedge pressure and cardiac index. 

These interaetions are established but, because aciclovir has such a wide therapeutie index, the authors of the study suggest that its interaetion with eimetidine is probably elinieally unimportant. It seems likely that no ehanges in the usual dosages of aciclovir or valaciclovir will be needed in patients also taking eimetidine. However, the UK manufaeturer states that eaution is required with high doses of valaciclovir, and that alternatives to eimetidine could be considered in this situation. No special precautions would seem necessary if cimetidine is used with famciclovir. 

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