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As the most commonly used illicit drug in the U.S., marijuana has been at the center of many politically charged debates because of its illegal status and widespread use.

At the request of the White House Office of National Drug Control Policy, the Institute of Medicine issued a report in 1999 summarizing researchers' review of all scientific evidence on marijuana, which concluded the drug's components have therapeutic qualities that warrant more clinical research. The institute did not recommend "crude marijuana," particularly when smoked, as medicine.

In the short term, marijuana's effects can include shortterm memory loss, distorted perception, a sense of euphoria or, in other users, paranoia and increased appetite.

Carcinogenic, or cancercausing, materials found in tobacco also are in marijuana, and smoking one marijuana cigarette can deposit three to four times as much tar in the respiratory tract as a single filtered tobacco cigarette, according to research from Dr. Donald Tashkin, a pulmonologist at the University of California at Los Angeles who has studied marijuana effects for 30 years.

As with tobacco cigarettes, smoking marijuana habitually has been shown to reduce respiratory function and harm the lining of the lungs, according to the institute report. Smoking marijuana regularly increases the risk of bronchitis and respiratory infection.

Supporters of medical marijuana contend that marijuana can be consumed in ways that don't involve smoking's harmful effects, including by eating it cooked in food products or using a vaporizer, which heats marijuana to the point of releasing THC, the active ingredient, but does not create toxin-containing smoke.

The largest studies of the relationship between marijuana and cancer have not found a link between marijuana and tobacco-related cancers. One large study, published in 1997 in the journal Cancer Causes and Control, found no association between marijuana and cancer, except in the case of prostate cancer in men who didn't smoke tobacco.

Another large study, led by Dr. Tashkin and published in Cancer Epidemiology, Biomarkers and Prevention in 2006, focused on lung and upper aerodigestive tract cancers. The study included more than 2,200 subjects and found no elevated risk among smokers of marijuana, even heavy users.

There is no record of any death from a marijuana overdose, according to the Institute of Medicine report, which also concluded, "Compared with alcohol, tobacco and several prescription medications, marijuana's abuse potential appears relatively small."

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