In about a dozen states, you can smoke a joint if you have cancer or HIV and meet certain conditions, like having a doctor's note.
In California, you can light up if you have just about anything — headaches, anxiety, epilepsy — and a physician's OK.
Illinois could join these states as legislators consider a bill that would allow patients to use marijuana as medicine as long as they have one of 14 conditions and illnesses, including cancer and Crohn's disease.
But interviews with scientists and physicians and a review of medical literature reveal scant evidence that marijuana is a safe and effective treatment for most of those 14.
A handful of uses in the bill — such as pain suffered by people with HIV and cancer — are supported by some solid scientific evidence. But none meet the standards, such as multiple large, well-designed clinical trials, required by the U.S. Food and Drug Administration in approving new drugs.
"What defines a medicine? And how do we bring medicines to market?" said Dr. Eric A. Voth, chairman of the Institute on Global Drug Policy. "And we do not bring them through the legislative vote process and say: 'Here, we deem this as medicine.'"
And yet the momentum across the U.S. leans toward legalizing medical marijuana, with bills being weighed from Pennsylvania to Ohio. On Tuesday, the District of Columbia Council passed a measure that legalizes medical cannabis. Advocates also are championing a change in federal law.
In the world of medicine, there's nothing quite like pot. It's a medicine sold with names like Haze ($160 an ounce at one California dispensary) and Grand Daddy Purple ($300 an ounce), and descriptions like "get lifted and be happy"; a stigmatized plant with therapeutic promise that few want to study because it remains illegal on the federal level and a drug that raises concerns because it often is smoked.
"We need more science and we need to treat it like a medicine," said Allan Young, a professor of psychiatry at the University of British Columbia who is conducting a trial examining the effect of chemicals in marijuana on bipolar disorder.
Advocates say they are only trying to decriminalize use of the plant by sick people who have failed to gain relief from pharmaceutical drugs. Under the Illinois bill, patients with permission from the state and a physician would be able to possess 2 ounces of dried marijuana or grow a small number of plants.
"These sick people are looking for compassion," said Dan Linn, executive director of the Illinois Cannabis Patients Association. "And if treatment includes cannabis, in Illinois, should we consider these people criminals?"
Illinois Rep. Lou Lang, a sponsor of the Illinois bill, said: "We have to think of this as a product, not a drug. Not as a menace. Nobody has ever died from an overdose of marijuana."
But there is reason to worry that marijuana could actually prove harmful for patients with some of the conditions it is supposed to treat.
Take glaucoma, a disease listed in the Illinois bill and often cited by advocates because marijuana can lower the pressure inside the eye. Increased eye pressure is a common feature of glaucoma, and can lead to damage of the optic nerve and blindness.
"They think that even if this unconventional therapy doesn't work that it can't possibly hurt their disease," said Dr. James Tsai, chairman of the Department of Ophthalmology and Visual Science at Yale University School of Medicine and chairman of the medical advisory board at The Glaucoma Foundation. "However, studies suggest that it might be, in fact, damaging to do so."
Marijuana only lowers pressure for several hours, requiring patients to continuously medicate day and night, glaucoma experts said. Failing to do so can lead to a rebound spike in eye pressure, which can be damaging. Marijuana also can lower blood pressure, which can damage the optic nerve.
In February, the Journal of Glaucoma ran an editorial warning against using pot to treat glaucoma.
Epilepsy is another disease commonly cited by advocates as treatable because marijuana is suspected to have anti-seizure properties. But ask epilepsy experts and they will tell another story.
"Statistically, there is no evidence that it is effective when used as a therapeutic agent and, besides, it has more side effects than other anti-seizure medications available," said neurologist Dr. Stephan Schuele, medical director of the Northwestern University Comprehensive Epilepsy Center.
There are serious concerns, said neurologist Dr. Alan Ettinger, epilepsy director of Neurological Surgery in Rockville Center, N.Y., and a member of the executive board of the national Epilepsy Foundation. First, he said, withdrawal among chronic users with epilepsy can cause severe exacerbations of the seizures.
And, he said, some individuals with epilepsy are struggling with depression, sleepiness and cognitive difficulties to begin with. Marijuana can compound these problems, he said.
Like glaucoma and epilepsy, research is mixed when it comes to another commonly cited medical use of marijuana — spasticity in people with multiple sclerosis, according to experts in the field.
One trial in Europe found that objective measures showed cannabis did not affect spasticity, even though patients thought it did, said neurologist Dr. Carlo Pozzilli, director of the Multiple Sclerosis Center in Rome, who has conducted research on cannabis and multiple sclerosis. It did, however, affect pain.
"This is the gap between what the patients say and what the doctor sees in terms of objectivity," Pozzilli said. "This is the big problem of cannabis as a therapeutic."
Advocates say marijuana can be a safe and effective alternative to FDA-approved pharmaceuticals, which can come with their own addiction problems and side effects. Mike Graham, a 47-year-old former restaurant manager from Manteno, Ill., said his degenerative disc disease left him bedridden with horrible nerve pain. "It is like getting hit by a baseball bat every time my heart beats," he said. "Boom. Boom. Boom. It doesn't stop."
The painkillers he was taking, including a morphine pump, failed to manage the pain but caused nausea and vomiting, he said. A hospice nurse suggested he try pot. He said it worked. Now he takes several puffs every three hours. "There is no euphoric feeling, but I can have a semblance of a life," said Graham, co-director of the Illinois chapter of Americans for Safe Access.
His story echoes that of the Rev. Wayne Dagit, a Michigan minister who runs a cannabis smokers club in Williamston, Mich., and is pushing for the Illinois bill.
Dagit said he awakens some mornings in so much pain that he can barely move. He has been prescribed oxycodone, a strong painkiller that can become addictive and takes 30 minutes to take effect. "But," he said, "if I can scoot up to the edge of the bed and do one hit (of marijuana), I wait four minutes and it is a euphoric effect and that is all I need."
Researchers long have been intrigued by marijuana's possibilities. Could cannabinoids, which affect areas of the brain that control movement, help people with multiple sclerosis control spasticity? Could the chemicals, which affect areas of the brain associated with stress, help veterans suffering from post-traumatic stress disorder?
In 1999, the Institute of Medicine released a report citing the promise of cannabinoids, recommending short-term use of marijuana for debilitating conditions like intractable pain or vomiting if, among other conditions, all other treatments have failed.
The report mostly calls for more research on uses of cannabis. But since that report, relatively little work has been done. Marijuana's status as an illegal drug, not just in the United States but across much of the world, has stymied researchers.
Marijuana, especially smoked marijuana, as a target of research faces serious obstacles, said internist Dr. Eric Larson, a co-author of the 1999 Institute of Medicine report. "It is an orphan drug, there is no U.S. company that is going to promote it and then there's the stigma," he said.
Larson said the social advocacy groups — pro and con — also make marijuana an unpopular choice for researchers. "Many traditional scientists will say, 'I don't want to have to deal with this sort of wild advocacy group where my science runs the risk of being expropriated for an agenda that isn't about discovery but rather about advocating a point of view.'"