Marijuana is addictive. Of course, not everyone who smokes marijuana will become addicted—that depends on a whole bunch of factors— including your family history (genes), the age you start using, whether you also use other drugs, your family and peer relationships, success in school, and so on. Repeated marijuana use can lead to addiction—which means that people have trouble controlling their drug use and often cannot stop even though they want to. Research shows that approximately 9 percent, or about 1 in 11, of those who use marijuana will become addicted. This rate increases to 17 percent, or about 1 in 6, if you start in your teens, and goes up to 25–50 percent among daily users.
Marijuana is unsafe if you are behind the wheel. Marijuana is the most common illegal drug involved in auto fatalities. It is found in the blood of around 14 percent of drivers who die in accidents, often in combination with alcohol or other drugs. Marijuana affects a number of skills required for safe driving—alertness, concentration, coordination, and reaction time—so it’s not safe to drive high or to ride with someone who’s been smoking. Marijuana makes it hard to judge distances and react to signals and sounds on the road. And combining marijuana with drinking even a small amount of alcohol greatly increases driving danger, more than either drug alone.
Marijuana is linked to school failure. Marijuana’s negative effects on attention, memory, and learning can last for days and sometimes weeks—especially if you use it often. Someone who smokes marijuana daily may have a ‘dimmed-down’ brain most or all of the time. Compared with their peers who don’t use, students who smoke marijuana tend to get lower grades and are more likely to drop out of high school. Research even shows that it can lower your IQ if you smoke it regularly in your teen years. Also, longtime marijuana users themselves report being less satisfied with their lives, having memory and relationship problems, poorer mental and physical health, lower salaries, and less career success.
High doses of marijuana can cause psychosis or panic when you're high. Some people experience an acute psychotic reaction (disturbed perceptions and thoughts, paranoia) or panic attacks while under the influence of marijuana. This usually goes away as the drug’s effects wear off. Scientists do not yet know if marijuana use causes lasting mental illness, although it can worsen psychotic symptoms in people who already have the mental illness schizophrenia, and it can increase the risk of long-lasting psychosis in some people.