by Melissa J Luther

Created on: September 21, 2009   Last Updated: October 01, 2009

Marijuana's active ingredients block pain signals.

U.S. federal law classifies marijuana as a Schedule I drug that has no medical use, making it a crime to possess or use it for any reason. However, 13 states have decriminalized possession of small amounts of marijuana for medical use.

Marijuana is made from the hemp plant, Cannabis sativa, and contains pharmacologically active chemicals called cannabinoids that act on cannabinoid receptors, which are found in the central nervous system and on immune system cells. Cannabinoid receptors exist because the human body produces its own cannabinoids, suggesting a legitimate biological function for these compounds.

> Marijuana in History <

Marijuana has a long history of use as a medicine, extending back at least twelve thousand years to China, where its uses included treatment of anxiety and pain. By the late thirteenth century, marijuana use had spread throughout Asia, the Middle East, Africa and Europe. The entire plant was used for both medicinal and non-medicinal purposes, ranging from weaving cloth to treating headaches and labor pains. Of course, it also had its recreational uses.

Marijuana was legal in the United States until the early 1900s, when states started banning it. In 1937 marijuana became illegal under federal law.

> Benefits and Limitations of Medical Marijuana <

Marijuana's most active cannabinoid, called tetrahydrocannabinal (THC), causes most of the drug's euphoric effects. THC is also the compound most studied for pain relief.

Despite the federal government's classification of marijuana as having no medical value, a 1999 report from the Institute of Medicine (IOM) concluded that cannabinoids reduce pain and nausea in patients with advanced cancer or AIDS. IOM recommended short-terms marijuana use for patients without other effective treatment options.

The same IOM report conceded that in most cases any benefits were outweighed by the risks of smoking. It went on to recommend development of synthetic cannabinoids and alternate delivery methods.

Many more studies provide evidence of the pain-relieving properties of marijuana, including a 2008 study published online in the Journal of Pain by University of California Davis researchers. This study found that patients suffering neuropathic pain experienced less pain after smoking marijuana. The report did also acknowledge the limitations of smoking as a delivery method.

Medical marijuana opponents maintain that crude plant preparations

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