Laboratory testing for drugs of abuse is often conducted in medical settings, with little consideration of the technical limitations and the potential for legal and social harm to the patient. We consider several technical problems associated with such testing, including the lack of chain-of-custody procedures, the possibility of false-positive results with screening immunoassays, and the infrequency of confirmatory testing. Important ethical issues arise because of the sensitive nature of drug test results, the ramifications of false-positive results, the limitations of confidentiality protection, and the practice of testing without the patient's knowledge. Taken together, these technical and ethical concerns suggest that drug testing policies in medical settings should specify which conditions require explicit informed consent, as well as create procedures for protecting this sensitive information.