WASHINGTON — Helping patients control pain and the nausea from cancer chemotherapy have been among the leading uses of medical marijuana.

Such patients are likely to be affected by the Supreme Court's decision today allowing federal prosecution of sick people who use the drug, regardless of state laws.

Marijuana has been used as a medical treatment for thousands of years, according to the Mayo Clinic. In recent years, marijuana and its chemical components have been studied in relation to illnesses ranging from cancer to glaucoma to multiple sclerosis.

The Institute of Medicine, a National Academy of Sciences component, reported in 1999 that "marijuana's active components are potentially effective in treating pain, nausea, the anorexia of AIDS wasting, and other symptoms, and should be tested rigorously in clinical trials."

There are scores of chemical compounds in the leaves, stem and seeds of the marijuana plant.

Drawing the most attention, the Mayo Clinic says, is THC, or delta-9-tetrahydrocannabinol, the part mainly responsible for the drug's mental effects and which may also help treat nausea and vomiting. A manufactured version, called Dronabinol or Marinol is used to treat nausea, though not all patients can keep it down.

In addition, the components cannabinol and cannabidol, which cause fewer mental effects but have some of THC's properties, have undergone study.

In the body, THC and other cannabinoids attach to two types of receptors on cells. CB1 receptors, found in the brain areas that control body movement, memory and vomiting, and CB2 receptors found on small numbers of cells elsewhere in the body, mainly in the immune system.

Some medical problems that have been studied for treatment with marijuana include:

- Glaucoma. This disease is associated with increased fluid pressure within the eye and can lead to vision loss and blindness.

The National Eye Institute reports that studies in the 1970s and 1980s showed marijuana lowered pressure within the eye when used orally, by injection or by smoking. However, the research indicated marijuana was no more effective than other drugs on the market.

- Cancer. Anticancer drugs can cause nausea and vomiting and marijuana has been studied as a means to reduce this side-effect, particularly Marinol, which is available by prescription.

The National Cancer Institute recommends other available anti-nausea drugs as first line therapy, but says its "scientists believe that synthetic THC may be appropriate for some cancer patients who have chemotherapy-induced nausea and vomiting that cannot be controlled by other antiemetic means."

- Multiple sclerosis. Some researchers believe marijuana may be useful in treating the pain of multiple sclerosis as well as protecting the nerves from damage, but results from studies have been mixed.

The National Multiple Sclerosis Society says, "There have been a large number of anecdotal reports from individuals who state that smoking marijuana has relieved some of their MS symptoms, including spasticity and pain. Studies completed thus far, however, have not provided convincing evidence that marijuana benefits people with MS."

- AIDS. Loss of appetite often occurs in AIDS patients and marijuana has been used as a stimulant to improve their eating.

The National Association of People with AIDS reports that in addition to appetite stimulation it is also useful for managing the side effects of drugs - side effects such as nausea.