SAN FRANCISCO -- Smoking marijuana eased HIV-related pain in some patients in a small study that nevertheless represented one of the few rigorous attempts to find out if the drug has medicinal benefits.

The Bush administration's Office of National Drug Control Policy quickly sought to shoot holes in the experiment.

The study, conducted at San Francisco General Hospital from 2003 to 2005 and published Monday in the journal Neurology, involved 50 patients suffering from HIV-related foot pain known as peripheral neuropathy. There are no drugs specifically approved to treat that kind of pain.

Three times daily for nearly a week, the patients smoked marijuana cigarettes machine-rolled at the National Institute of Drug Abuse, the only legal source for the drug recognized by the federal government.

Half the patients received marijuana, while the other 25 received placebo cigarettes that lacked the drug's active ingredient, tetrahydrocannabinol. Scientists said the study was the first one published that used a comparison group, which is generally considered the gold standard for scientific research.

Thirteen patients who received marijuana told doctors their pain eased by at least a third after smoking pot, while only six of those smoking placebos said likewise. The marijuana smokers reported an average pain reduction of 34 percent, double the drop reported by the placebo smokers as measured with a widely accepted pain scale.

"These results provide evidence that there is measurable medical benefit to smoking cannabis for these patients," said Dr. Donald Abrams, the University of California, San Francisco professor who led the study.

Many critics of smoked marijuana agree THC has promise as a painkiller, but they argue the smoke itself is harmful.

"People who smoke marijuana are subject to bacterial infections in the lungs," said David Murray, chief scientist at the Office of National Drug Control Policy. "Is this really what a physician who is treating someone with a compromised immune system wants to prescribe?"

Murray also questioned the statistical relevance of study with just 50 participants in the test.

Dr. Mark Ware, a researcher at McGill University in Montreal conducting similar tests, defended Abrams' study as sound and statistically reliable.

The study is one of the few human tests in a research field nearly devoid of federal funding.

"This is a valid medicine and I want safe access to my medication," said Diana Dodson, a 50-year-old grandmother of five who participated in the test in 2004.

California and 10 other states have passed laws legalizing marijuana for medicinal purposes, but the federal government considers it a dangerous drug, like cocaine or heroin. The U.S. Supreme Court ruled in 2005 that state laws do not protect users from the federal ban.

To conduct the test, Abrams needed authorization from eight academic and government agencies, including the Drug Enforcement Administration.

The study cost about $1 million and was paid for by the University of California Center for Medicinal Cannabis Research, which has sponsored several smoked marijuana tests.