Cannabis leaves. Computer artwork of cannabis leaves arranged in circles. The cannabis (Cannabis sativa) or hemp plant is native to central Asia. Its leaves and flowering tops are used to make cannabis resin (marijuana), an illegal drug in most countries. Smoking cannabis can cause mental relaxation, perceived slowing of time and heightened senses. Large doses may result in panic attacks and illusions. Prolonged use may lead to sterility, lung damage and psychological problems. Smoking marijuana can help to relieve the symptoms of multiple sclerosis, a disease of the central nervous system. However, medicinal use remains both controversial and illegal.