Marijuana (cannabis) is the most commonly used illicit substance. This drug impairs short-term memory and learning, the ability to focus, and coordination. It also increases heart rate, can harm the lungs, and may increase the risk of psychosis in vulnerable people. Research suggests that when regular marijuana use begins in the teen years, addiction is more likely: 1 in 6 users, compared to 1 in 9 among adults. In addition, recent research suggests that heavy cannabis use that starts in the teen years is associated with declines in IQ scores in adulthood.