Premature thelarche: a possible adverse effect of cimetidine treatment.

An 18-month-old girl is reported in whom marked unilateral breast enlargement occurred after 4 weeks of cimetidine therapy. After withdrawal of the drug the enlargement rapidly disappeared. This observation points to cimetidine as a possible cause of premature thelarche. Cimetidine, a selective H2 receptor blocking agent, is known to cause gynaecomastia in males. This effect seems related to elevated plasma oestrogens, gonadotropins, or to binding of the drug to androgen receptors. Ranitidine, a much more potent selective H2 receptor blocker, does not cause gynaecomastia and seems therefore preferable to cimetidine.