It is believed that some 7.7 million Americans suffer from post-traumatic stress disorder, or PTSD. Yet current treatments for the sometimes devastating anxiety disorder have limited efficacy. Now, a Yale University professor, who recently made a presentation of his research at a conference in Oakland, contends that there is strong evidence that the main ingredient in marijuana, THC, not only helps alleviate symptoms of PTSD, but also may ultimately help cure it.

Yale associate professor of psychiatry R. Andrew Sewell has an impeccable résumé: He's the former chief resident in neuropsychiatry at the University of Massachusetts School of Medicine, and his research is grounded in some pretty solid animal studies — which is the only reason why the United States government is permitting his multi-year lab test on about 120 people, specifically, veterans with intractable cases of PTSD.

PTSD can show up months or years after a person is exposed to trauma, and veterans are especially susceptible to it after experiencing the horrors of war. Symptoms include extended periods of flashbacks, agitation, and anxiety, which can be triggered by a thought, word, or object. It can lead to depression, insomnia, and abuse of alcohol and other drugs. Sewell sees PTSD regularly in his work with veterans at a Yale-affiliated center in West Haven, Connecticut.

But existing treatments for PTSD, including antidepressants like Prozac, Paxil, and Zoloft, have "limited efficacy," Sewell said, and side effects can include thoughts about suicide. "It helps a bit with emotional numbing, but it doesn't really do much for the other things," he said.

Sewell said the most effective cure for PTSD is "exposure therapy," which can be brutal. It involves revisiting the trauma over and over again until the memories of it are no longer overwhelming. Lots of veterans drop out of the twelve-week course at the National Center for PTSD during week three, when they have to write down what happened to them. "You have to be exposed to something that's very unpleasant that you've spent a lot of time and effort walling off in your own brain," he explained.

Sewell's theory is that the main active molecule in cannabis, THC, can speed up and enhance exposure therapy by increasing something called "extinction learning." Extinction learning is the process of learning new information that opposes the old. Sewell gave an example of what can happen to soldiers who return home after driving in Iraq. About 20 percent of Iraq war vets have PTSD.

"[In Iraq] you don't have to watch the road," he noted. "What you have to keep your eye on is little bits of garbage on the side of the road [because of improvised explosive devices] ... and also bridges are huge. ... Somebody can drop a frag grenade from the bridge and run off. ... Soldiers come back [to the United States]; they find they can't drive anymore because they're always crashing. ... They're scared of garbage. ... Bridges freak them out," he said.

After six months, most vets' symptoms go away on their own. New information overwrites the old. "These soldier reflexes never leave you completely, but they die away to the extent that you can live a normal life," he said. Except if you have PTSD, the reflexes don't die away. "Suppose some part of your reptile brain thinks if you walk under a bridge you're going to die," Sewell said. "Life becomes very hard."

Sewell said PTSD and other anxiety disorders might hinge on a defect in the extinction learning process in the brain at the cellular level. Our brain cells "talk" to each other using a chemical system, and the system's main switch is called the CB1 receptor. If you knock out the CB1 in mice or apes and subject them to shocks, they get PTSD and they're permanently shell-shocked. Conversely, stimulating the CB1 receptor enhances extinction learning in mammals. And guess what drug is really good at stimulating the CB1? Pot.

"It's not like they'd have to keep taking cannabis the rest of their life," Sewell said. "We're talking about a cure."

Here's the lab model: Sewell gets vets with PTSD and a control group, and then subjects all of them to electric shocks when they see a certain geometric pattern. They all learn to associate some squares with a zap. Then he takes away the shocks. Normal, healthy people in the control group quickly learn no more shocks are coming. They stop sweating and producing stress hormones. But PTSD sufferers are stuck. "They still respond to the shapes as though there will be a shock even when there is not a shock coming," he explained. "That's kind of what you see in the diseases of PTSD as well."

Sewell then gives each veteran about one milligram of pure THC intravenously ("Oh yeah they get high," he said) and sees if the vets' extinction learning re-starts. They should be able to learn no more shocks are coming and relax. Eventually, vaporized THC could be coupled with exposure therapy to treat PTSD, or other social anxiety disorders.

Sewell presented his ongoing study in Oakland on April 18 at the Multidisciplinary Association for Psychedelic Studies (MAPS) conference. MAPS scientist Dr. Sue Sisley called the approach "compelling." Thousands of vets are already using marijuana to manage symptoms of PTSD, she said, even as the federal government blocks research on the plant.

Marijuana science remains a political battlefield, Sewell agreed. "If you want to show cannabis is helpful for veterans and their sleeping, you're not going to get to," Sewell said. "If you want to show how ... it's a gateway drug, you name it, they'll throw money at you."

Update: A previous version of this story misstated the amount of THC Sewell have his study participants. It was one milligram, not one gram.