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Q: What are the long-term effects of marijuana?

A: While all of the long-term effects of marijuana use are not yet known, there are studies showing serious health concerns. For example, a group of scientists in California examined the health status of 450 daily smokers of marijuana but not tobacco. They found that the marijuana smokers had more sick days and more doctor visits for respiratory problems and other types of illness than did a similar group who did not smoke either substance.

Findings so far show that the regular use of marijuana or THC may play a role in cancer and problems in the respiratory, immune, and reproductive systems.


It is hard to find out whether marijuana alone causes cancer because many people who smoke marijuana also smoke cigarettes and use other drugs. Marijuana smoke contains some of the same cancer-causing compounds as tobacco, sometimes in higher concentrations. Studies show that someone who smokes five joints per week may be taking in as many cancer-causing chemicals as someone who smokes a full pack of cigarettes every day.

Tobacco smoke and marijuana smoke may work together to change the tissues lining the respiratory tract. Marijuana smoking could contribute to early development of head and neck cancer in some people.

Reproductive system

Heavy marijuana use can affect hormones in both males and females, so it can affect sexual characteristics and reproductive function. Heavy doses of the drug may delay the onset of puberty in young men. Marijuana also can have adverse effects on sperm production.

Among women, regular marijuana use can disrupt the normal monthly menstrual cycle and inhibit the discharge of eggs from the ovaries (ovulation).

Immune system

The immune system protects the body from many agents that cause disease. It is not certain whether marijuana damages the immune system of people. But both animal and human studies have shown that marijuana impairs the ability of T-cells in the lungs' immune defense system to fight off some infections. People with HIV and others whose immune system is impaired should avoid marijuana use.

Respiratory system

Someone who smokes marijuana regularly may have many of the same respiratory problems that tobacco smokers have. They have symptoms of daily cough and phlegm (chronic bronchitis) and more frequent chest colds. Continuing marijuana smoking can lead to abnormal function of the lungs and airways. Scientists have found signs of lung tissue injured or destroyed by marijuana smoke.

Q: What about pregnancy: Will smoking marijuana hurt the baby?

A: Doctors advise pregnant women not to use any drugs because they might harm the growing fetus.

Some scientific studies have found that babies born to marijuana users were shorter, weighed less, and had smaller head sizes than those born to mothers who did not use the drug. Smaller babies are more likely to develop health problems. Other scientists have found effects of marijuana that resemble the features of fetal alcohol syndrome. There are also research findings that show nervous system problems in children of mothers who smoked marijuana.

Researchers are not certain whether a newborn baby's health problems, if they are caused by marijuana, will continue as the child grows.

Q: What happens if a nursing mother uses marijuana?

A: When a nursing mother uses marijuana, some of the THC is passed to the baby in her breast milk. This is a matter for concern, since the THC in the mother's milk is much more concentrated than that in the mother's blood. One study has shown that the use of marijuana by a mother during the first month of breastfeeding can impair the infant's motor development (control of muscle movement).

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