After anecdotal reports of marijuana's providing antiemetic activity in cancer chemotherapy patients refractory to standard agents, orally administered delta 9-tetrahydrocannabinol (THC) was formally studied by a number of investigators. In six of seven well-controlled studies, orally administered THC was a superior antiemetic agent compared with control agents. The THC toxic effects are notable but manageable. Patients rarely require hospitalization after the development of THC-induced dysphorias. However, serious toxic effects are uncommon and the most frequently noted effects are somnolence, conjunctivitis, and tachycardias. Because certain subgroups of patients are more prone to have toxicities develop, careful selection of the candidates to receive this agent is mandatory. Overall, the benefits of orally administered THC use represent a major advance in antiemetic therapy.