In 1972, the U.S. Congress decided that marijuana should be added to the list of Schedule I of the Controlled Substances Act because there was, supposedly, no legitimate medical use for it. Schedule I drugs include heroin, MDMA (better known as Ecstasy), LSD, psilocybin (aka "magic mushroom") , peyote and its derivative, mescaline, methaqualone (Quaaludes--powerful sedatives once prescribed by doctors, but now prohibited), and other substances.
Since then, there have been a number of studies done that seem tofind legitimate medical applications for cannabis and/or its psychoactive ingredient, THC. These include: restoring appetite to cancer and HIV/AIDS patients, relieving chronic, severe pain due to disease or injury including migraines, reducing the symptoms of glaucoma and epilepsy, treating multiple sclerosis and muscle spasms, reducing nausea due to chemotherapy, and other medical conditions.
The 1999 Institute of Medicine's (IOM) report on "Marijuana and Medicine" noted that, in fact, marijuana has a number of legitimate medical uses. But IOM also notes that when marijuana is smoked, it exposes the user to many of the same hazards as smoking tobacco (lung cancer, emphysema, asthma, etc.) because of the high concentration of tars and other harmful substances in the smoke.
Ingesting marijuana in an edible form (cookies, brownies, other foods) eliminates the risks of smoking. Nonetheless, the drug can have powerful psychoactive effects on the user that may result in undesired psychological effects, such as feelings of paranoia or panic, hallucinatory visions, and the like.
For a more complete look at the pros and cons of medical marijuana, we recommend that you go to ProCon.org, a nonprofit, nonpartisan website that maintains up-to-date information on the pluses and minuses of the medical marijuana issue.