Yahoo answers:
According to the NORML (the National Organization for the Reform of Marijuana Laws), ten states currently legalize or decriminalize the medical use of marijuana in some manner.
Since 1996, Alaska, Arizona, California, Colorado, Hawaii, Maine, Nevada, Oregon, and Washington have passed legislation that removes state-level penalties from marijuana use by patients who have a doctor's recommendation. In addition, Maryland passed a medical marijuana affirmative defense law in 2003. This dictates that if a person is arrested for marijuana use but is found to be using marijuana out of medical necessity, she or he will only face a modest fine.
These state laws specify that marijuana can be used to treat diseases such as arthritis, cachexia, cancer, chronic nervous system disorders, chronic pain, Crohn's disease, epilepsy and other seizure disorders, glaucoma, HIV or AIDS, multiple sclerosis and other muscle spasticity disorders, and help patients cope with severe migraines, severe nausea, and the side effects of chemotherapy. The requirements for patients and doctors to be protected by these laws vary from state to state.
Unfortunately, the legal status of such measures is debatable. The U.S. government has challenged the California law in several cases. Federal law does not recognize a medical use for marijuana and maintains that the drug is a controlled and banned substance under all circumstances.