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The Health Effects of Marijuana

Negative Health Effects Are Numerous

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Updated March 18, 2014

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.

Although legalization activists and many marijuana users believe smoking pot has no negative effects, scientific research indicates that marijuana use can cause many different health problems.

Marijuana is the most commonly used illicit drug in the United States. When smoked, it begins to effect users almost immediately and can last for one to three hours. When it is eaten in food, such as baked in brownies and cookies, the effects take longer to begin, but usually last longer.

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Short-Term Effects

The short-term effects of marijuana include: Sometimes marijuana use can also produce anxiety, fear, distrust, or panic.

Do you experience withdrawals when you quit smoking weed? Take the Marijuana Withdrawal Symptom Quiz.

Effects on the Brain

The active ingredient in marijuana, delta-9 tetrahydrocannabinol or THC, acts on cannabinoid receptors on nerve cells and influences the activity of those cells. Some brain areas have many cannabinoid receptors, but other areas of the brain have few or none at all. Many cannabinoid receptors are found in the parts of the brain that influence pleasure, memory, thought, concentration, sensory and time perception, and coordinated movement.

When high doses of marijuana are used, usually when eaten in food rather than smoked, users can experience the following symptoms:

Effects on the Heart

Within a few minutes after smoking marijuana, the heart begins beating more rapidly and the blood pressure drops. Marijuana can cause the heart beat to increase by 20 to 50 beats per minute, and can increase even more if other drugs are used at the same time.

Because of the lower blood pressure and higher heart rate, researchers found that users' risk for a heart attack is four times higher within the first hour after smoking marijuana, compared to their general risk of heart attack when not smoking.

Effects on the Lungs

Smoking marijuana, even infrequently, can cause burning and stinging of the mouth and throat, and cause heavy coughing. Scientists have found that regular marijuana smokers can experience the same respiratory problems as tobacco smokers do, including:

Most marijuana smokers consume a lot less cannabis than cigarette smokers consume tobacco, however the harmful effects of smoking marijuana should not be ignored. Marijuana contains more carcinogenic hydrocarbons than tobacco smoke and because marijuana smokers typically inhale deeper and hold the smoke in their lungs longer than tobacco smokers, their lungs are exposed to those carcinogenic properties longer, when smoking.

What About Cancer?

Although one study found that marijuana smokers were three times more likely to develop cancer of the head or neck than non-smokers, that study could not be confirmed by further analysis.

Because marijuana smoke contains three times the amount of tar found in tobacco smoke and 50 percent more carcinogens, it would seem logical to deduce that there is an increased risk of lung cancer for marijuana smokers. However, researchers have not been able to definitively prove such a link because their studies have not been able to adjust for tobacco smoking and other factors that might also increase the risk.

Studies linking marijuana smoking to lung cancer have also been limited by selection bias and small sample size. For example, the participants in those studies may have been too young to have developed lung cancer yet. Even though researchers have yet to "prove" a link between smoking pot and lung cancer, regular smokers may want to consider the risk.

Other Health Effects

Research indicates that THC impairs the body's immune system from fighting disease, which can cause a wide variety of health problems. One study found that marijuana actually inhibited the disease-preventing actions of key immune cells. Another study found that THC increased the risk of developing bacterial infections and tumors.

Effects of Exposure During Pregnancy

Several studies have found that children born to mothers who used marijuana during pregnancy exhibit some problems with neurological development. According to those studies, prenatal marijuana exposure can cause:

See Also: The Health Effects of Other Drugs

Sources:

Chuder, Eric C., "Neuroscience for Kids: Marijuana," University of Washington, Revised 2008.

SAMHSA's National Clearinghouse for Alcohol & Drug Information. "Tips for Teens: The Truth About Marijuana," Revised May 2004.

National Institute on Drug Abuse. "Marijuana: Facts Parents Need to Know." Revised August 2007.

Tetrault, J.M., et al. "Effects of marijuana smoking on pulmonary function and respiratory complications: a systematic review." Archives of Intern Medicine. February 2007.

Mehra, R, et al. "The association between marijuana smoking and lung cancer: a systematic review." Archives of Internal Medicine. July 2006.

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