Delta-9-tetrahydrocannabinol (THC) and prochlorperazine (Compazine) were found to be equally efficacious in reducing nausea and vomiting associated with cancer chemotherapy across a wide range of chemotherapeutic regimens and tumor types. Both drugs were administered orally one hour before chemotherapy, then every four hours for a total of four doses. Compazine was administered in a fixed dose of 10 mg; THC was administered by body surface area (BSA): BSA less than 1.4 m2 = 7.5 mg; BSA 1.4-1.8 m2 = 10- mg; and BSA greater than 1.8 m2 = 12.5 mg. Two hundred and fourteen subjects (75% of whom had previously received Compazine with varying results) were evaluated employing a double-blind, crossover design. Additional parameters evaluated were study drug effects on appetite, food intake, mood, activity, relaxation, interaction, and concentration. There were significant drug effects with THC: less ability to concentrate (P less than 0.01), less social interaction (P less than 0.05), and less activity (P less than 0.05). There were no significant differences between the two drugs in the level of food intake or appetite. Patients of all ages did equally well on both drugs. Neither past marijuana use nor past Compazine use were related to study the drug efficacy. Those patients who correctly identified their THC cycle did better on THC versus those who could not correctly identify which antiemetic they had received (P less than 0.05). There were more drug-related effects associated with THC, but these did not reduce the patients' preference for the drug, and were associated with nausea reduction (P less than 0.05).