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 an amber or gold colored glassy solid when cold, which becomes viscous and sticky if warmed.
Female cannabis plants contain at least 113 cannabinoids, including cannabidiol (CBD), thought to be the major anticonvulsant that helps people with multiple sclerosis; and cannabichromene (CBC), an anti-inflammatory which may contribute to the pain-killing effect of cannabis.
THC, or tetrahydrocannabinol, is the primary ingredient in marijuana responsible for the high. But getting high is not all that it's good for. THC also has a wide range of medical benefits, and is commonly reported to relieve pain, nausea, and depression, among many other things.
While THC is still illegal in the U.S. and most countries around the world, synthetic versions of the chemical have been legally prescribed for decades. The first THC-based pharmaceutical, a pill sold as Marinol (scientific name: dronabinol), was developed by a company called Unimed Pharmaceuticals with funding from the National Cancer Institute.