Marijuana has been used for over 2 centuries. Its major psychoactive constituent, delta-9-tetrahydrocannabinol (THC) was isolated in 1964 and first used to control nausea and vomiting during chemotherapy in the 1970s. THC has cardiovascular, pulmonary and endocrinological effects as well as actions on the central nervous system. Alterations in mood, memory, motor coordination, cognitive ability, sensorium, spatial- and self-perception are commonly experienced. The precise antiemetic mechanism is unknown. THC and nabilone act at a number of sites within the central nervous system. Cannabinoids have also been shown to inhibit prostaglandin synthesis in vitro. In controlled clinical trials, THC is superior to placebo and prochlorperazine in antiemetic effectiveness. Effectiveness of THC correlates to a 'high' experienced by the patient. A variety of chemotherapy regimens respond to THC including high-dose methotrexate and the doxorubicin, cyclophosphamide, fluorouracil combination. Cisplatin is more resistant. Side effects are generally well tolerated but may limit THC use in the elderly or when high doses are administered. Nabilone, a synthetic cannabinoid, is also an effective antiemetic which is more active than prochlorperazine in preventing chemotherapy-induced emesis, including cisplatin-containing regimens. Side effects are similar to THC and may be dose-limiting. Levonantradol, another synthetic cannabinoid, is an effective antiemetic. It may provide more flexibility in the outpatient setting since it can be administered orally or intramuscularly. Most side effects are mild except for dysphoria which may be dose-limiting.