Despite the Obama administration's tacit support of more liberal state medical marijuana laws, the federal government still discourages research into the medicinal uses of smoked marijuana.

That may be one reason that—even though some patients swear by it—there is no good scientific evidence that legalizing marijuana's use provides any benefits over current therapies.

Lyle E. Craker, a professor of plant sciences at the University of Massachusetts, has been trying to get permission from federal authorities for nearly nine years to grow a supply of the plant that he could study and provide to researchers for clinical trials.

But the Drug Enforcement Administration has refused, even after the agency's own administrative law judge ruled in 2007 that Craker's application should be approved, and even after Attorney General Eric H. Holder Jr. in March ended the Bush administration's policy of raiding dispensers of medical marijuana that comply with state laws.

“;All I want to be able to do is grow it so that it can be tested,”; Craker said in comments echoed by other researchers.

Marijuana is the only major drug for which the federal government controls the only legal research supply and for which the government requires a special scientific review.

“;The more it becomes clear to people that the federal government is blocking these studies, the more people are willing to defect by using politics instead of science to legalize medicinal uses at the state level,”; said Rick Doblin, executive director of a nonprofit group dedicated to researching psychedelics for medical uses.

On Monday, his last full day in office, Gov. Jon S. Corzine of New Jersey signed a measure passed by the Legislature last week that made the state the 14th in the nation to legalize the use of marijuana to help with chronic illnesses.

The measure was pushed by a loose coalition of patients suffering from chronic illnesses like Lou Gehrig's disease and multiple sclerosis who said marijuana eased their symptoms.

Studies have shown convincingly that marijuana can relieve nausea and improve appetite among cancer patients undergoing chemotherapy. Studies also prove that marijuana can alleviate the aching and numbness that patients with HIV and AIDS suffer.

There are strong hints that marijuana may ameliorate some of the neurological problems associated with such degenerative diseases as multiple sclerosis, said Dr. Igor Grant, director of the Center for Medicinal Cannabis Research at the University of California, San Diego.

But there is no good evidence that legalizing the smoking of marijuana is needed to provide these effects. The Food and Drug Administration in 1985 approved Marinol, a prescription pill of marijuana's active ingredient, THC Although a few small-scale studies done decades ago suggest that smoked marijuana may prove effective when Marinol does not, no conclusive research has confirmed this finding.

And Marinol is no panacea. There are at least three medicines that in most patients provide better relief from nausea and vomiting than Marinol, studies show.

Buddy Coolen, 31, of Warwick, R.I., said he tried or continued to use some of those medicines. “;Smoking for me is as good as any medicine I have,”; he said.

Eight years ago, Coolen contracted gastroparesis and cyclic vomiting syndrome. He lost 50 pounds and, despite being 5 foot 11, weighed 120 pounds.

His doctors gave him myriad anti-emetics, many of which he still takes. They also prescribed Marinol, but it did not work for him, Coolen said.

“;My stepdad is old school and was really against marijuana, but then he saw what it did for me and totally changed his way of thinking,”; Coolen said.

Some doctors and law enforcement officials say such anecdotes should not drive public policy. Dr. Eric Braverman, medical director of a multispecialty clinic in Manhattan, said legalizing marijuana was unnecessary and dangerous since Marinol provided the medicinal effects of the plant. “;Our society will deteriorate,”; he said.

Patients who call Braverman's clinic are, when put on hold, told that the clinic may prescribe supplements and other alternative treatments that have even less scientific justification than marijuana. Braverman said such alternatives rendered marijuana unnecessary, but his embrace of alternatives is a reminder that medicine has long been driven by more than science.

About 20 percent of drug prescriptions are written for uses that are not approved by federal drug regulators; about half of the nation's adults regularly take supplements; herbal and homeopathic remedies are popular.

The nation's growing embrace of medical marijuana has stemmed from these alternative traditions.

The University of Mississippi has the nation's only federally approved marijuana plantation. If they wish to investigate marijuana, researchers must apply to the National Institute on Drug Abuse to use the Mississippi marijuana and must get approvals from a special Public Health Service panel, the Drug Enforcement Agency and the Food and Drug Administration.

But federal officials have repeatedly failed to act on marijuana research requests in a timely manner or have denied them, according to a 2007 ruling by an administrative law judge at the Drug Enforcement Agency. While refusing to approve a second marijuana producer, the government allowed the University of Mississippi to supply Mallinckrodt, a drugmaker, with enough marijuana to eventually produce a generic version of Marinol.

“;As the National Institute on Drug Abuse, our focus is primarily on the negative consequences of marijuana use,”; said Shirley Simson, a spokeswoman for the drug abuse institute, known as NIDA. “;We generally do not fund research focused on the potential beneficial medical effects of marijuana.”;

The Drug Enforcement Agency said it was just following NIDA's lead. “;DEA has never denied a research registration for marijuana and/or THC if NIDA approved the protocols for that individual entity,”; a supervisory special agent, Gary Boggs, said in an e-mailed statement.

Researchers investigating LSD, Ecstasy and other illegal drugs can use any of a number of suppliers licensed by the Drug Enforcement Agency, Doblin said. And if a researcher wants to use a variety of marijuana that the University of Mississippi does not grow—and there are many with differing medicinal properties—they are out of luck, Doblin said.

Law enforcement tends to emphasize the abuse potential of medicines without regard to their positive effects. Bureaucratic battles between the DEA and the FDA over the availability of narcotics—highly effective but addictive medicines—have gone on for decades.

So medical marijuana may never have good science underlying its use. But for patients in desperate need, the ethics of providing access to the drug are clear, said Dr. Richard Payne, a professor of medicine and divinity and director of the Institute for Care on the End of Life at Duke Divinity School.

“;It's not a great drug,”; he said, “;but what's the harm?”;