There are only a limited number of drugs proven to be human teratogens including thalidomide, isotretinoin, coumarin derivates, valproic acid, and folate antagonists. In some cases, the combination of 2 drugs may increase the teratogenic risk. The risk of birth defects may also vary with the time at which the drug is administered during pregnancy and the dose. There are some examples of drugs in which the dose has proven to be a major determinant of their teratogenicity in humans. There is more safety information for older than for newer drugs. Proactive teratogen risk counseling should include a critical appraisal of all available data including the consequences of the untreated maternal condition.