Factors influencing preference for THC vs. Compazine (prochlorperazine) as an antiemetic agent during cancer chemotherapy were studied in 139 patients who received both medications in a double-blind randomized crossover design trial. Nausea reduction was the main determinant of preference. THC preference was associated with more, rather than fewer, drug-related side effects than Compazine, particularly sedation. Patients who reported being anxious or depressed did not experience accentuation of their mood states with either regime. Mood effects, nausea reduction, incidence of side effects, and drug preference were the same in patients under and over 50 years of age. Patients with a history of illicit drug use reported fewer side effects from THC, but reported no difference in drug preference or nausea reduction compared to those patients without a history of illicit use.