Dosage regimen of cimetidine reviewed. Possible drug accumulation after multiple oral doses.

The doses of cimetidine recommended differ in children, especially those with cystic fibrosis. These dosage regimens were derived from single-dose pharmacokinetic studies of the drug. Some authors showed, however, that after administration of repeated oral doses of cimetidine in healthy adults and children with cystic fibrosis, the elimination half-life of the drug was markedly prolonged. In view of the ability of cimetidine to inhibit metabolism of other drugs, it is suggested that the parent compound and/or it is metabolite(s) may inhibit its own metabolism during a prolonged course of treatment. Enterohepatic recirculation of the drug and/or its metabolite(s) may also contribute to prolongation of its elimination. One should therefore be cautious in using single-dose pharmacokinetic parameters to calculate repeated dose regimens and expected plasma steady-state concentrations.