THC cannabis drug. Molecular graphic of tetra- hydrocannabinol (THC), a psychoactive drug derived from the hemp plant (Cannabis sativa). THC has the formula C21.H30.O2. The atoms are shown as spheres and are colour-coded: carbon (red), hydrogen (yellow) and oxygen (green). THC is always taken as a component of marijuana or hashish. Marijuana (the dried leaves and flowering heads) and hashish (the resin) are usually smoked, although they can be mixed with foods and eaten. Smoking produces more rapid, albeit shorter-lived, effects, which include relaxation and euphoria. THC is an illegal drug in most countries.