The green, pungent leaves of the cannabis plant — known as “marijuana,” “weed” and a host of other names — maintain a stronghold as the most popular illicit drug among U.S. teens. When smoked, marijuana releases THC, a potent psychoactive chemical. This makes the user feel relaxed, heightens their senses and has a mild hallucinogenic effect. It also causes paranoia and impaired motor function and is highly addictive.