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  more concerned about abuse than potential benefits  has refused, even after the agency’s own administrative law judge ruled in 2007 that Dr. 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,” Dr. Craker said in comments echoed by other researchers.