Tetrahydrocannabinol (THC) is one of at least 113 cannabinoids identified in cannabis. THC is the principal psychoactive constituent of cannabis. With chemical name (−)-trans-Δ⁹-tetrahydrocannabinol, the term THC also refers to cannabinoid isomers. Like most pharmacologically active secondary metabolites of plants, THC is a lipid found in cannabis, assumed to be involved in the plant's self-defense, putatively against insect predation, ultraviolet light, and environmental stress.
THC, along with its double bond isomers and their stereoisomers, is one of only three cannabinoids scheduled by the UN Convention on Psychotropic Substances (the other two are dimethylheptylpyran and parahexyl). It was listed under Schedule I in 1971, but reclassified to Schedule II in 1991 following a recommendation from the WHO. Based on subsequent studies, the WHO has recommended the reclassification to the less-stringent Schedule III. Cannabis as a plant is scheduled by the Single Convention on Narcotic Drugs (Schedule I and IV). It is specifically still listed under Schedule I by US federal law under the Controlled Substances Act for having "no accepted medical use" and "lack of accepted safety". However, dronabinol is a synthetic form of THC approved by the FDA as an appetite stimulant for people with AIDS and antiemetic for people receiving chemotherapy. The pharmaceutical formulation dronabinol is an oily resin provided in capsules available by prescription in the United States, Canada, Germany, and New Zealand.