“I just don’t think the evidence is there for these long lists,” said Dr. Molly Cooke, a professor of medicine at the University of California, San Francisco, who helped research a position paper on cannabis for the American College of Physicians. “It’s been so hard to study marijuana. Policy makers are responding to thin data.”

Even some advocates of medical marijuana acknowledge that the state laws legalizing it did not result from careful reviews of the medical literature.

“I wish it were that rational,” said Mitch Earleywine, chairman of the executive board of directors for Norml, a national marijuana advocacy group. Dr. Earleywine said state lawmakers more often ask themselves, “What disease does the person in a wheelchair in my office have?”

Research into marijuana’s effects is thin not because of a lack of scientific interest, but chiefly because the federal government has long classified it as a Schedule 1 drug with “no currently accepted medical use.” Scientists who want to conduct studies must register with the Drug Enforcement Administration, submit an investigational new drug application to the Food and Drug Administration for human trials, and win approval from the Department of Health and Human Services or one of the National Institutes of Health. The National Institute on Drug Abuse is the only supplier of legal, research-grade marijuana.

The legal and administrative hoops make it hard for investigators to start the randomized, placebo-controlled trials that are the gold standard of medical research and the basis for determining which drugs are effective, at what doses, and in which patients.