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updated 11/29/2010 5:32:48 PM ET 2010-11-29T22:32:48

Smoking marijuana can suppress the body's immune system, which explains why pot-smokers are more susceptible than non-smokers to certain cancers and infections, according to a new study.

This effect of marijuana is due to chemicals in the drug that fire up the body's production of immune system cells called myeloid-derived suppressor cells. While most immune system cells are protective — fighting infections and cancers to keep a person healthy — these cells suppress the immune system, keeping it in check, according to the study.

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"Cannabis is one of the most widely used drugs of abuse worldwide, and it is already believed to suppress immune functions, making the user more susceptible to infections and some types of cancer," Prakash Nagarkatti, a microbiology and pathology professor at the University of South Carolina, said in a statement.

Nagarkatti and his colleagues focused their study on cannabinoids, compounds found in the cannabis plant, to see how they affected immune suppression and tumor growth. Their study included the pain-relieving compound delta-9 tetrahydrocannabinol (THC).

The researchers injected one group of mice with THC and compared them with a group not injected with the compound.

The mice injected with THC had more of the immune-suppressing cells than the mice who didn't receive THC, according to the study.

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The researchers found that marijuana triggered the production of a massive number of myeloid-derived suppressor cells — leading to immune suppression and cancer growth — by activating cells that respond to the cannabinoids found in marijuana, researchers said.

Cancer patients have more of these cells than healthy people. The suppressor cells can even hinder cancer therapy and promote cancer growth, the study said.

"Marijuana cannabinoids present us with a double edged sword," Nagarkatti said, because they can cause increased susceptibility to cancer and infections, but they can also open the door to opportunities to treat disorders where a suppressed immune system is beneficial, such as arthritis, multiple sclerosis, lupus and hepatitis.

The study is published in the December issue of the European Journal of Immunology.

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