WASHINGTON – As the nation's only truly legal supplier of marijuana, the U.S. government keeps tight control of its stash, which is grown in a 12-acre fenced garden at the University of Mississippi in Oxford.

Part of the crop is shipped to Research Triangle Institute in North Carolina, where it's rolled into cigarettes, all at taxpayer expense.

Even though Congress has long banned marijuana, the operation is legitimate. It's run by the National Institute on Drug Abuse, part of the U.S. Department of Health and Human Services, which doles out the pot for federally approved research.

While U.S. officials defend their monopoly, critics say the government is hogging all the pot and giving it mainly to researchers who want to find harms linked to the drug.

U.S. officials say the federal government must be the sole supplier of legal marijuana in order to comply with a 1961 international drug-control treaty. But they admit they've done relatively little to fund pot research projects looking for marijuana's benefits, following their mandate to focus on abuse and addiction.

"We've been studying marijuana since our inception. Of course, the large majority of that research has been on the deleterious effects, the harmful effects, on cognition, behavior and so forth," said Steven Gust, special assistant to the director at the National Institute on Drug Abuse.

Most Americans back legal pot

With polls showing a majority of Americans supporting legalization, pot backers say the government should take a more evenhanded approach. The National Institute on Drug Abuse and the White House drug czar have become favorite targets to accuse of bias, with both prohibited by Congress from spending money to promote legalization.

Some critics hope the situation will change; federal officials last week approved a University of Arizona proposal that will let researchers try to determine whether smoking or vaporizing marijuana could help veterans with post-traumatic stress disorder. The researchers can provide the equivalent of two joints per day for 50 veterans.

It was a long time coming.

Suzanne Sisley, clinical assistant professor of internal medicine and psychiatry at the University of Arizona's medical school, said the Health and Human Services Department waited more than three years to approve the project after it was first sanctioned by the Food and Drug Administration. She said the extra federal review should be scrapped and that approval by the FDA should be sufficient for a study to proceed.

"It's indefensible," she said. "The only thing we can assume is that it is politics trumping science."

After the long delay, Sisley said she hopes to launch the project soon, after getting the marijuana from the National Institute on Drug Abuse. She said pressure by veterans helped get the project approved.

Dan Riffle, director of federal policies for the Marijuana Policy Project, a pro-legalization group, said President Obama should remove research barriers. The legalization of marijuana "is inevitable" and more studies are needed, he said.

"That is exactly why federal law and policies shouldn't tie the hands of scientists by favoring certain kinds of research over others," Riffle said.

Research is not being blocked

The national institute's Gust said the federal government is open to the idea of looking for more medical applications for marijuana and that it's a "red herring" to say that his agency is blocking research.

"This is an untruth that's been put out there by certain groups, and quite frankly I wonder if it's not having the perverse effect of actually decreasing the amount of applications and interest in research," Gust said.

National Institute on Drug Abuse officials said they gave more than $30 million in government grants to finance 69 marijuana-related research projects in 2012, a big jump from the 22 projects that received less than $6 million in 2003. While the institute would not provide exact figures, Gust said it has funded at least five to 10 projects examining possible medical applications.

The institute also provides marijuana for privately funded projects approved by the Health and Human Services Department. Of the 18 research applicants who requested marijuana from 1999 to 2011, 15 got approval, officials said.