PIP: Many reports on the excretion of drugs in human breastmilk were prepared during a period when pharmacologic assays lacked precision, sensitivity, and specificity and are based on accidental single case reports. This article critically reviews the evidence underlying drugs that are frequently contraindicated in nursing mothers. Specific medications and preparations examined include aspirin, bromides, iodides, thiouracil, chlorpromazine, lithium carbonate, benzodiazepines, ergot alkaloids, metronidazole, marijuana, chlormycetin, tetracyclines, antimetabolites, cathartics, anticoagulants, oral contraceptives, and radioactive materials. The review suggests that very few drugs are known to be absolutely contraindicated in nursing mothers. It is possible, however, that ingestion of small amounts of drugs may have subtle, undetected effects on infant growth and development. Thus, physicians are advised to prescribe drugs to lactating women only when there is a clear indication for its use. The pharmacokinetics of the drug should be taken into account in order to have the lower peak level in the milk at the time of feeding. Information on drugs that are not generally considered to be contraindicated in nursing mothers is presented in table form.