Tetrahydrocannabinol (THC, dronabinol by INN), or more precisely its main isomer (−)-trans-Δ⁹-tetrahydrocannabinol, is the principal psychoactive constituent (or cannabinoid) of cannabis. It can be a clear, amber or gold colored glassy solid when cold, which becomes viscous and sticky if warmed.
Like most pharmacologically-active secondary metabolites of plants, THC in Cannabis is assumed to be involved in self-defense, perhaps against herbivores. THC also possesses high UV-B (280–315 nm) absorption properties, which, it has been speculated, could protect the plant from harmful UV radiation exposure.
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 signed by the US Congress in 1970.
A pharmaceutical formulation is available by prescription in the U.S. and Canada under the brand name Marinol.
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