Marijuana has been used for centuries by doctors all over the world. Some conditions for which marijuana was formerly a common treatment, such as simple pain relief, are now treatable by other, more effective, medications. In other cases, marijuana, alone or in combination with other drugs, remains an effective treatment.

• Nausea reduction in cancer and AIDS patients. The most common medical application of marijuana is for the reduction of extreme nausea caused by cancer chemotherapy and AIDS treatment. Patients facing such treatments often find that just a small amount of marijuana - whether inhaled, baked into foods or consumed in liquids - can immediately quell nausea or even prevent its onset. Several scientific studies in the late 1970s and early 1980s showed marijuana's value for reducing cancer patients' nausea.
• Increasing appetite. Marijuana also reportedly increases appetite for patients with nausea or other conditions, permitting more normal food intake and preventing dangerous weight loss. This is particularly important to patients with AIDS "wasting syndrome."
• Reducing eye pressure in glaucoma patients. Glaucoma is a progressive disease of the eye which can lead to blindness. It results from a buildup of pressure within the eye. Marijuana reduces the pressure within the eye, holding off some of the damage.
• Controlling muscle spasms, seizures and chronic muscular pain. Marijuana is also used medically by patients with epilepsy, multiple sclerosis, arthritis, spinal cord injuries and other conditions which are characterized by muscle spasticity, seizures and severe chronic pain.
• Alzheimer's disease - Research done by the Scripps Research Institute in California shows that the active ingredient in marijuana, THC, prevents the formation of deposits in the brain associated with Alzheimer's disease. THC was found to prevent an enzyme called acetylcholinesterase from accelerating the formation of "Alzheimer plaques" in the brain more effectively than commercially marketed drugs. THC is also more effective at blocking clumps of protein that can inhibit memory and cognition in Alzheimer’s patients, as reported in Molecular Pharmaceutics.  
• Lung cancer and chronic obstructive pulmonary disease. THC has been found to reduce tumor growth in common lung cancer by 50 percent and to significantly reduce the ability of the cancer to spread, say researchers at Harvard University, who tested the chemical in both lab and mouse studies. The researchers suggest that THC might be used in a targeted fashion to treat lung cancer. Breast cancer
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• According to a 2007 study at the California Pacific Medical Center Research Institute, cannabidiol (CBD) may stop breast cancer from spreading throughout the body.[79] The scientists believe[weasel words] their discovery may provide a non-toxic alternative to chemotherapy while achieving the same results minus the painful and unpleasant side effects. The research team says that CBD works by blocking the activity of a gene called Id-1, which is believed to be responsible for a process called metastasis, which is the aggressive spread of cancer cells away from the original tumor site.
• Investigators at Columbia University published clinical trial data in 2007 showing that HIV/AIDS patients who inhaled cannabis four times daily experienced substantial increases in food intake with little evidence of discomfort and no impairment of cognitive performance. They concluded that smoked marijuana has a clear medical benefit in HIV-positive patients. In another study in 2008, researchers at the University of California, San Diego School of Medicine found that marijuana significantly reduces HIV-related neuropathic pain when added to a patient's already-prescribed pain management regimen and may be an "effective option for pain relief" in those whose pain is not controlled with current medications. Mood disturbance, physical disability, and quality of life all improved significantly during study treatment.[82] Despite management with opioids and other pain modifying therapies, neuropathic pain continues to reduce the quality of life and daily functioning in HIV-infected individuals. Cannabinoid receptors in the central and peripheral nervous systems have been shown to modulate pain perception. No serious adverse effects were reported, according to the study published by the American Academy of Neurology.[83]
• Brain cancer - A study by Complutense University of Madrid found the chemicals in marijuana promotes the death of brain cancer cells by essentially helping them feed upon themselves in a process called autophagy. The research team discovered that cannabinoids such as THC had anticancer effects in mice with human brain cancer cells and in people with brain tumors. When mice with the human brain cancer cells received the THC, the tumor shrank. Using electron microscopes to analyze brain tissue taken both before and after a 26- to 30-day THC treatment regimen, the researchers found that THC eliminated cancer cells while leaving healthy cells intact. The patients did not have any toxic effects from the treatment; previous studies of THC for the treatment of cancer have also found the therapy to be well tolerated. However, the mechanisms which promote THC's tumor cell–killing action are unknown.
• Opioid dependence - Injections of THC eliminate dependence on opiates in stressed rats, according to a research team at the Laboratory for Physiopathology of Diseases of the Central Nervous System (France) in the journal Neuropsychopharmacology. Deprived of their mothers at birth, rats become hypersensitive to the rewarding effect of morphine and heroin (substances belonging to the opiate family), and rapidly become dependent. When these rats were administered THC, they no longer developed typical morphine-dependent behavior. In the striatum, a region of the brain involved in drug dependence, the production of endogenous enkephalins was restored under THC, whereas it diminished in rats stressed from birth which had not received THC. Researchers believe the findings could lead to therapeutic alternatives to existing substitution treatments. In humans, drug treatment subjects who use cannabis intermittently are found to be more likely to adhere to treatment for opioid dependence. Historically, similar findings were reported by Clendinning, who in 1843 utilized cannabis substitution for the treatment of alcoholism and opium addiction and Birch, in 1889, who reported success in treating opiate and chloral addiction with cannabis.
  (related; Religious uses of Marijuana; Michelle Rainey, Prop 215)