Study subjects did feel high, an effect that varied among individuals. Marijuana also affected thinking, shown as problems with tasks of memory and complicated reasoning after the strongest marijuana cigarettes were used. Potentially problematic, these effects were tolerated by subjects -- no one opted out of the study because they couldn't think straight.

Grant says it's important to have a choice of treatments because not everyone responds to or can tolerate the available drugs. Antidepressants are used for neuropathic pain but cause dry mouth, constipation and urinary problems, and must be avoided by people with conditions such as glaucoma. Others can't take aspirin-like drugs. "Having an alternative compound is always good," Grant says.

Multiple sclerosis


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Patients with multiple sclerosis suffer muscle spasms, pain and tremor. Anecdotal reports suggest that marijuana may be helpful, but controlled studies are few. One, presented at an April meeting, had 51 multiple sclerosis patients smoke 0% or 4% THC marijuana cigarettes daily for three days. Intensity of spasms was reduced by 32% and pain ratings by 50% after smoking marijuana, compared with 2% and 22% reductions after placebo cigarettes. Five subjects withdrew, citing side effects: feeling too high, dizzy or fatigued.

Other studies in patients with multiple sclerosis used a cannabis extract that can be taken orally. In a 2007 European Journal of Neurology study, nearly half of 184 patients experienced at least 30% improvement in muscle spasms.

But a 2004 Neurology paper showed no reduction in objective measures of arm tremor with cannabis extract, although five subjects out of 13 reported feeling improvement. This might have resulted from mood-altering effects of the drug or from some aspect of tremor not measured.

Nausea

A 2008 review published in the European Journal of Cancer Care analyzed 30 clinical studies using cannabinoid drugs synthesized in the lab and concluded that they were better than standard antinausea drugs in alleviating the nausea and vomiting that accompanies chemotherapy. One such drug is Marinol, a THC preparation approved by the Food and Drug Administration for precisely this purpose.

Survey studies suggest that some people with HIV smoke marijuana to counteract nausea caused by antiretroviral therapy. Researchers at the UC Center for Medicinal Cannabis Research have tried to study the effect of smoked marijuana on nausea and vomiting in patients undergoing chemotherapy but have struggled to enroll enough subjects, Grant says.

Bruce Mirken, director of communications for the Marijuana Policy Project -- a group that lobbies for the decriminalization of marijuana -- says he is all for research on the chemical components in marijuana with the goal of making more-purified and perhaps more-targeted drugs that do not deliver a "high," but does not see "criminalizing use of that plant by people who are ill when you are making its main psychoactive ingredient legal in the form of a very expensive pill."

Tom Riley, a spokesman for the White House Office of National Drug Control Policy, says marijuana advocates are seeking a free pass. "They want to be exempted from the regular [drug] approval process," he says.

Con marijuana: Damaging habit?

Marijuana is the most widely used illicit drug in the country -- an estimated 25 million Americans smoked it within the last year and close to 100 million have smoked it at least once in their life, according to the most recent National Survey on Drug Use and Health by the federal Substance Abuse and Mental Health Services Administration.

Rates and severity of marijuana addiction pale in comparison to that of legal addictive drugs, alcohol and nicotine, according to the Advisory Council on the Misuse of Drugs, a panel of independent experts advising the British government, in a rare head-to-head, scientific comparison.

Yet, the fact is, recreational use can lead to addiction, and inhaling marijuana smoke is unhealthful for the lungs. Some researchers argue that marijuana may predispose heavy users to mental illnesses such as psychosis and depression.

How big are these risks and how should they be measured against health benefits? "The FDA has ruled that marijuana has no medical benefits, but its harms are well known and proven," says Tom Riley, a spokesman for the White House Office of National Drug Control Policy, referring to an April 2006 statement released by the FDA and several other federal agencies concluding that smoking marijuana was not of medicinal use.

For comparison's sake, Riley cites the prescription drug Vioxx. The FDA, he notes, pulled Vioxx off the market in spite of its proven efficacy, because it created problems in a small number of people.

Then, too, the number of people adversely affected by marijuana use is large, Riley says. "There are more teens in drug treatment for marijuana dependence than for alcohol or any other drug," he says.

Marijuana is a Schedule 1 drug by the Drug Enforcement Administration's Controlled Substances Act, a classification reserved for drugs carrying the highest risk for addiction and no medical benefit.

Scientists have reviewed the weed's risks and find them to be real, but small. Ten years ago, the Institute of Medicine reviewed the scientific evidence about marijuana at the request of the Office of National Drug Control Policy. The 1999 report states that, "except for the harms associated with smoking, the adverse effects of marijuana use are within the range of effects tolerated for other medications."