Medical marijuana inspires strong opinions, but what does science say?
Depending on whom you ask, marijuana is a dangerous drug that should be kept illegal alongside heroin and PCP, or it's a miracle herb with a trove of medical benefits that the government is seeking to deny the public -- or something in between: a plant with medical uses and drawbacks, worth exploring.

As the political debates over medical marijuana drag on, a small cadre of researchers continues to test inhaled marijuana for the treatment of pain, nausea and muscle spasms.

All drugs have risks, they point out -- including ones in most Americans' medicine cabinets, such as aspirin and other pain-relievers or antihistamines such as Benadryl. Doctors try to balance those risks against the potential for medical good -- why not for marijuana as well, they ask.

The truth, these researchers say, is that marijuana has medical benefits -- for chronic-pain syndromes, pain, multiple sclerosis, AIDS wasting syndrome and the nausea that accompanies chemotherapy -- and attempts to understand and harness these are being hampered. Also, they add, science reveals that the risks of marijuana use, which have been thoroughly researched, are real but generally small.

Dr. Donald Abrams, chief of hematology and oncology at San Francisco General Hospital and professor of clinical medicine at UC San Francisco, says he sees cancer patients in pain, not eating or sleeping well, experiencing nausea and vomiting from treatment, and being depressed about their situation. He says he is glad that he lives in California, where use of medical marijuana is allowed by state law, although federal officials continue to raid cannabis dispensaries in the state and scrutinize practices of physicians who specialize in writing cannabis recommendations for patients.

"I can talk to patients about medicinal cannabis [and] I'm often recommending it to them for these indications," Abrams says.

Pro marijuana: A balm for pain

Medical marijuana use has a history stretching back thousands of years. In prebiblical times, the plant was used as medicinal tea in China, a stress antidote in India and a pain- reliever for earaches, childbirth and more throughout Asia, the Middle East and Africa.

In recent decades, medical researchers have investigated marijuana's effects on various kinds of pain -- from damaged nerves in people with HIV, diabetes and spinal cord injury; from cancer; and from multiple sclerosis. Marijuana has also been hypothesized to help with nausea induced by chemotherapy and antiretroviral therapy, and with severe loss of appetite as seen in people with the AIDS wasting syndrome.

The weed's actions are due to the active ingredients tetrahydrocannabinol (THC) and some 60 other cannabinoids, which mimic the action of chemicals -- known as endogenous cannabinoids -- that exist naturally in the brain. Those cannabinoids activate receptors in our nerves, triggering physiological responses.

A legal prescription form of THC (Marinol) exists, yet researchers say it's far from a perfect drug. Taken orally, its absorption is highly variable and unpredictable and often delayed, says Dr. Igor Grant, a UC San Diego psychiatrist who directs the university's Center for Medicinal Cannabis Research. "Smoking is a very efficient way to deliver THC," he says.

As a result of its federally illegal status, medicinal use of marijuana is restricted to carefully vetted clinical research studies or to patients in states such as California that have passed laws to allow for personal medical use. Research on the medicinal use of marijuana relies on government-issued marijuana cigarettes, which come in different strengths and are supplied by the National Institute on Drug Abuse.

The UC Center for Medicinal Cannabis Research in San Diego helps coordinate clinical studies to investigate the safety and effectiveness of marijuana. Here's what they've found.

Neuropathic pain

Recent research suggests that marijuana can assuage this chronic-pain syndrome in which burning sensations occur and simple touch can feel like hurt. It is unaffected by aspirin-like drugs and fairly resistant to stronger analgesics such as opiates.

In a 2007 study on neuropathic pain related to HIV infection, 50 patients smoked marijuana cigarettes three times a day or marijuana cigarettes from which active ingredients had been extracted. Subjects then rated their pain on a scale ranging from "no pain" to "worst pain imaginable." The results, published in the journal Neurology, showed a 34% reduction in ratings of pain in the marijuana group compared with 17% in the placebo group over five days of treatment.

Another study in 44 patients reported in June in the Journal of Pain found that marijuana alleviated neuropathic pain arising from a variety of conditions, including spinal-cord injury and diabetes. Participants smoked marijuana on a set schedule -- first two puffs, then three puffs an hour later, then four puffs an hour after that -- from a single cigarette containing either 0%, 3.5%, or 7% THC. Average pain ratings before smoking were 55 on a 100-point scale and decreased by 46% in both treatment groups and by 27% in the placebo group one hour after the last puff.

Analgesic drugs are often tested against experimentally induced pain. Such studies have been conducted for marijuana too. In one 2007 report in the journal Anesthesiology, 15 healthy volunteers received skin injections with capsaicin -- the chemical behind that fiery spice in chile peppers -- and then smoked different-strength marijuana cigarettes. The medium dose, with a 4% THC concentration, lessened the burning pain.

These three pain studies all concluded that smoked marijuana can bring relief to sufferers of neuropathic pain comparable to other analgesic drugs. It is not a cure, Grant says: "It's like other pain medicines, you have to keep taking it."