THC cannabis drug crystals. Coloured scanning electron micrograph of crystals of tetrahydrocannabinol (THC, brown) on the surface of processed marijuana (blue green). THC is the main psychoactive substance found in the plant Cannabis sativa, which can be smoked to produce rapid, shortlived effects of relaxation and euphoria. Marijuana is an illegal drug in many countries, although a synthetic form of THC is used to treat various illnesses, such as MS (multiple sclerosis).
