The antiemetic efficacy of im levonantradol, a synthetic cannabinoid, given at a dose of 1 mg every 4 hours, was compared to oral delta-9-tetrahydrocannabinol (THC) given at a dose of 15 mg every 4 hours in a double-blind crossover study. Twenty-six patients receiving emetogenic cancer chemotherapy were evaluated. For each drug, 28% of treated patients had no nausea. The median number of emetic episodes with levonantradol was 2.0 versus 3.0 for THC (P = 0.06). Side effects occurred in 91.7% and 97.3% of levonantradol and THC patients, respectively, with drowsiness and dizziness most commonly seen. Side effects were generally well-tolerated, with only 13.9% of levonantradol and 21.6% of THC patients discontinuing treatment because of side effects. Levonantradol appears to be at least as effective an antiemetic as THC and is the only cannabinoid available for parenteral use.