WASHINGTON - The Food and Drug Administration said Thursday that "no sound scientific studies" supported the medical use of marijuana, contradicting a 1999 review by a panel of highly regarded scientists.
The announcement inserts the health agency into yet another fierce political fight.
Susan Bro, an agency spokeswoman, said Thursday's statement resulted from a past combined review by federal drug enforcement, regulatory and research agencies that concluded "smoked marijuana has no currently accepted or proven medical use in the United States and is not an approved medical treatment."
Ms. Bro said the agency issued the statement in response to numerous inquiries from Capitol Hill but would probably do nothing to enforce it.
"Any enforcement based on this finding would need to be by D.E.A. since this falls outside of F.D.A.'s regulatory authority," she said.
Eleven states have legalized medicinal use of marijuana, but the Drug Enforcement Administration and the director of national drug control policy, John P. Walters, have opposed those laws.
A Supreme Court decision last year allowed the federal government to arrest anyone using marijuana, even for medical purposes and even in states that have legalized its use.
Congressional opponents and supporters of medical marijuana use have each tried to enlist the F.D.A. to support their views. Representative Mark Souder, Republican of Indiana and a fierce opponent of medical marijuana initiatives, proposed legislation two years ago that would have required the food and drug agency to issue an opinion on the medicinal properties of marijuana.
Mr. Souder believes that efforts to legalize medicinal uses of marijuana are a front for efforts to legalize all uses of it, said Martin Green, a spokesman for Mr. Souder.
Tom Riley, a spokesman for Mr. Walters, hailed the food and drug agency's statement, saying it would put to rest what he called "the bizarre public discussion" that has led to some legalization of medical marijuana.
The Food and Drug Administration statement directly contradicts a 1999 review by the Institute of Medicine, a part of the National Academy of Sciences, the nation's most prestigious scientific advisory agency. That review found marijuana to be "moderately well suited for particular conditions, such as chemotherapy-induced nausea and vomiting and AIDS wasting."
Dr. John Benson, co-chairman of the Institute of Medicine committee that examined the research into marijuana's effects, said in an interview that the statement on Thursday and the combined review by other agencies were wrong.
The federal government "loves to ignore our report," said Dr. Benson, a professor of internal medicine at the University of Nebraska Medical Center. "They would rather it never happened."
Some scientists and legislators said the agency's statement about marijuana demonstrated that politics had trumped science.
"Unfortunately, this is yet another example of the F.D.A. making pronouncements that seem to be driven more by ideology than by science," said Dr. Jerry Avorn, a medical professor at Harvard Medical School.
Representative Maurice D. Hinchey, a New York Democrat who has sponsored legislation to allow medicinal uses of marijuana, said the statement reflected the influence of the Drug Enforcement Administration, which he said had long pressured the F.D.A. to help in its fight against marijuana.
A spokeswoman for the Drug Enforcement Administration referred questions to Mr. Walters's office.
The Food and Drug Administration's statement said state initiatives that legalize marijuana use were "inconsistent with efforts to ensure that medications undergo the rigorous scientific scrutiny of the F.D.A. approval process."
But scientists who study the medical use of marijuana said in interviews that the federal government had actively discouraged research. Lyle E. Craker, a professor in the division of plant and soil sciences at the University of Massachusetts, said he submitted an application to the D.E.A. in 2001 to grow a small patch of marijuana to be used for research because government-approved marijuana, grown in Mississippi, was of poor quality.
In 2004, the drug enforcement agency turned Dr. Craker down. He appealed and is awaiting a judge's ruling. "The reason there's no good evidence is that they don't want an honest trial," Dr. Craker said.
Dr. Donald Abrams, a professor of clinical medicine at the University of California, San Francisco, said he had studied marijuana's medicinal effects for years but had been frustrated because the National Institutes of Health, the leading government medical research agency, had refused to finance such work.
With financing from the State of California, Dr. Abrams undertook what he said was a rigorous, placebo-controlled trial of marijuana smoking in H.I.V. patients who suffered from nerve pain. Smoking marijuana proved effective in ameliorating pain, Dr. Abrams said, but he said he was having trouble getting the study published.
"One wonders how anyone" could fulfill the Food and Drug Administration request for well-controlled trials to prove marijuana's benefits, he said.
Marinol, a synthetic version of a marijuana component, is approved to treat anorexia associated with AIDS and the nausea and vomiting associated with cancer drug therapy.
GW Pharmaceutical, a British company, has received F.D.A. approval to test a sprayed extract of marijuana in humans. Called Sativex, the drug is made from marijuana and is approved for sale in Canada. Opponents of efforts to legalize marijuana for medicinal uses suggest that marijuana is a so-called gateway drug that often leads users to try more dangerous drugs and to addiction.
But the Institute of Medicine report concluded there was no evidence that marijuana acted as a gateway to harder drugs. And it said there was no evidence that medical use of marijuana would increase its use among the general population.
Dr. Daniele Piomelli, a professor of pharmacology at the University of California, Irvine, said he had "never met a scientist who would say that marijuana is either dangerous or useless."
Studies clearly show that marijuana has some benefits for some patients, Dr. Piomelli said.
"We all agree on that," he said.
Note: The FDA Statement is at www.fda.gov/bbs/topics/NEWS/2006/NEW01362.html
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