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.