Gynaecomastia associated with cimetidine.

Gynaecomastia has occurred unilaterally or bilaterally in five out of 25 male duodenal ulcer patients after more than four months treatment with cimetidine 1.6 g daily. All elected to continue treatment to 12 months and their breast enlargement regressed rapidly and disappeared after stopping treatment. During treatment all patients were found to have normal concentrations of plasma testosterone and oestradiol, and serum prolactin was normal in the two patients measured. Excision biopsy of the subareolar tissue in one patient revealed histology typical of the florid stage of gynaecomastia. Blockade of androgen-responsive receptors receptors in the target organ appears to be the most likely mechanism involved.