Marijuana Use and Its Effects

Marijuana, which comes from the hemp plant Cannabis sativa, is the most frequently used illegal drug in the U.S. About 4% of American adults smoke pot at least once a year. Roughly 1% of adults abuse pot, and one in 300 have a pot addiction.

Most people smoke the plant's dried leaves, flowers, stems, and seeds. But marijuana also can be mixed into food or brewed as a tea. Marijuana goes by many street names, including pot, weed, and herb. Hash, a concentrated form of the drug, is short for hashish.

The rates of marijuana smoking in adults have remained stable since the 1990s. However, the rates of addiction to pot have risen significantly over that same period. And, according to recent government studies, as many as 30% of today's teenagers are smoking marijuana.

Occasional marijuana use is rarely seriously harmful, but smoking pot has important medical effects.

Physiological Effects of Marijuana

The active ingredient in marijuana is THC. That's short for delta-9-tetrahydrocannabinol.

THC is rapidly absorbed after smoking pot. Within minutes, THC and the other substances in marijuana smoke cause short-term medical effects.

Signs of using marijuana include:

• rapid heart rate
• increased blood pressure
• increased rate of breathing
• red eyes
• dry mouth
• increased appetite, or "the munchies"
• slowed reaction time

These effects are reduced after three or four hours. However, marijuana hangs around in your system for as long as 24 hours after smoking. The lingering effects mean you're impaired for several hours after the high wears off.

Psychological Effects of Marijuana

The main psychological effect of smoking pot is euphoria. Getting high or "stoned" is the reason most pot smokers use marijuana.

Other short-term psychological effects of pot include:

• distorted sense of time
• paranoia
• magical or "random" thinking
• anxiety and depression

These psychological signs of using pot also generally ease after a few hours. But residual effects can last through the next day.

Risks of Marijuana Use

The risks of smoking marijuana go up with heavy use. Although the link has never been proven, many experts believe heavy pot smokers are at increased risk for lung cancer.

Heavy marijuana use lowers men's testosterone levels and sperm count and quality. Pot could decrease libido and fertility in some heavy-smoking men.

Contrary to what many pot smokers may tell you, marijuana is addictive, at least psychologically. Even among occasional users, one in 12 can feel withdrawal symptoms if they can't get high when they want to. Among heavy pot smokers, the rates of dependence are higher.

Many experts also believe that marijuana is physically addictive. Symptoms of withdrawal from pot might include:

• aggression
• anxiety
• depressed mood
• decreased appetite

Is pot a "gateway" drug? In other words, does smoking marijuana make someone more likely to try cocaine, heroin, ecstasy, and other "hard" drugs? The jury is still out on this one. It's true that pot smokers are more likely to use other drugs after trying marijuana. What's not clear is whether smoking pot causes further drug use or if people who start smoking pot are just more likely to try drugs in general.

If you're wondering how long marijuana stays in your system after smoking, it depends on how often you smoke. Light users -- those who smoke pot once in a while -- will have a negative drug screen after a marijuana-free week. Heavy users -- sometimes called "stoners" -- may continue testing positive for a month after last smoking pot.