Dr. Donald Abrams believes cannabis can ease pain, but that it isn't a cure. "Most of the patients that I've cared for have used it, probably smoked it," he said. "If cannabis cured cancer, I'd have a lot more survivors." America Tonight

NIDA is also less inclined to support studies that are looking at possible benefits of marijuana use, according to Dr. Donald Abrams, a cancer and integrative medicine specialist at the University of California, San Francisco.

“Generally, they will only fund your study if you're looking at cannabis as a substance of abuse or something that's harmful,” he said.

Abrams is one of the few physicians to ever receive federal permission to study marijuana and he did so by studying abuse potential and medical risks, or by receiving limited funding from state sources. In his work, he found, as have the few researchers who have looked at the issue, that the cannabinoids in marijuana can ease inflammation and pain.

“There isn’t a day that goes by that I don’t see a patient that has nausea, loss of appetite, pain, depression, insomnia, and I could recommend one medicine to treat all five of those instead of writing a prescription for five different medicines,” said Abrams. "I’ve seen it for the past 32 years, time and time again."

Abrams believes marijuana can improve the therapeutic effects of other pain medications and allow for lower doses of addictive opiate drugs, like Vicodin, Oxycontin and Percocet, which are tied to 16,000 overdose deaths in the U.S. each year.

In his experience, Abrams said marijuana seems to improve the painkilling qualities of opiate drugs, possibly reducing the patient’s risk of pill overdose.

"I called the DEA and I said, 'How about cannabis?' And they said we would never be able to fund a clinical trial looking at a Schedule 1 substance as an adjunct to opiates,” Abrams said, and then shrugged. "Science has been impeded by the politics."

Marijuana research regularly splashes front pages. But those studies often involve small sample sizes and the results aren't repeated, such as a 2012 Duke study that purported to find a link between heavy marijuana use and IQ decline among teenagers. At the same time, small studies and anecdotal stories suggest marijuana, or its components, could treat diverse conditions, including cancer, Crohn's Disease, epilepsy, HIV and Alzheimer's disease. But funding, legal issues and stigma make it uniquely difficult to set up the necessary clinical trials.

Meanwhile, at least five states, including California, which accounts for 12 percent of the U.S. population, appear poised to vote on ballot measures in 2016 that would legalize the drug, only widening the gap between the number of people who use marijuana and a true scientific understanding of its effects on the body.

Correction: January 16, 2015

An earlier version of this article stated that MAPS' study about the effects of marijuana on PTSD-afflicted veterans is the first time an organization has received federal permission to study the potential benefits of smoking marijuana. Since 1999, the federal government has cleared three other organizations for research grade marijuana to study its possible therapeutic effects: San Mateo County Health Department, Health Canada and the University of California at San Diego. The MAPS study is the first to receive permission to provide federal marijuana to PTSD-afflicted veterans.