Marijuana is the most commonly abused illegal drug in the United States. It affects memory, judgment, and perception. A dry, shredded green/brown mix of flowers, stems, seeds, and leaves from the hemp plant (Cannabis sativa), marijuana is usually smoked. The main psychoactive (affecting the mind and behavior) chemical in marijuana is THC (delta-9-tetrahydrocannabinol). The membranes of certain nerve cells contain protein receptors that bind to THC. When THC binds to these receptors in the brain, a series of cellular reactions ultimately lead to the high that users experience.
Since marijuana can affect judgment, its use can lead to risky behaviors, resulting in exposure to sexually transmitted diseases like HIV, the virus that causes AIDS. THC also can interfere with the normal functioning of the hippocampus—the part of the brain that controls learning and memory. It can cause difficulties in processing information and may make it hard to remember things that happened recently. Grades can suffer. Judgment and decision making also can be affected, increasing a person’s chances of getting into situations where he or she might be at risk—or where he or she might put others at risk.
THC also interferes with the normal functioning of the cerebellum, which gives a person balance and coordination, and the basal ganglia, which controls movement. Since marijuana can affect coordination and the perception of time and speed, a person’s performance in everything from playing sports to driving may suffer. In fact, marijuana is the most prevalent illegal drug found in impaired drivers and crash victims involved in "drugged driving" accidents.
For more information check out:
"Marijuana: Facts for Teens"
"NIDA Research Report Series"
"NIDA for Teens"
"NIDA InfoFacts: Drugged Driving"
Learn more about impairment from drug abuse