Prolactin responses to cimetidine.

1 An intravenous injection of cimetidine 400 mg to four healthy male subjects resulted in high blood concentrations of cimetidine and a rapid three-fold increase in serum prolactin. 2 This effect was prevented by pretreatment with bromocriptine. 3 No increase in prolactin followed a single oral dose of cimetidine 800 mg administered to a different group of healthy male subjects. The mean peak blood concentration of cimetidine was less than 20% of that achieved with 400 mg i.v. 4 Only isolated reports have been received of gynaecomastia or galactorrhoea occurring during cimetidine treatment. In three of seven cases studied there was associated hyperprolactinaemia. This may be an idiosyncratic response at the lower blood concentration of cimetidine associated with oral therapeutic dose regimens.