Although there's nothing definitive—yet—there's growing evidence that marijuana is effective in treating PTSD.

Experiments with animals involving tetrahydrocannabinol (THC), the active ingredient in marijuana, have shown the chemical to be effective on the areas of the brain that regulate fear and anxiety. Unsurprisingly, researchers—and undoubtedly many laymen—have suspected that marijuana might be effective in treating PTSD, but there was little evidence about its usefulness until fairly recently. According to NPR's Jon Hamilton, it wasn't until 2002, when German scientists published a study that showed that mice produce chemicals in their brains called cannabinoids to modulate fear, that things really got rolling. Hamilton explains the study's implications and influence: 

There are two common sources of cannabinoids. One is the brain itself, which uses the chemicals to regulate a variety of brain cells. The other common source is Cannabis sativa, the marijuana plant.

So in recent years, researchers have done lots of experiments that involved treating traumatized mice with the active ingredient in pot, tetrahydrocannabinol (THC), [Emory University's Dr. Kerry] Ressler says. And in general, he says, the mice who get THC look "less anxious, more calm, you know, many of the things that you might imagine."

While marijuana seems to be effective in treating the symptoms of PTSD—as with mice, people who've taken THC tend to get "less anxious, more calm"—there is little evidence that it has permanent effects. According to Ressler, "prolonged exposure seems to make brain cells less sensitive to the chemical." 

There's also the issue of pot's unwanted side effects. The National Institute on Alcohol Abuse and Alcoholism's Andrew Holmes told Hamilton that, "You may indeed get a reduction in anxiety. But you're also going to get all of these unwanted effects," including short-term memory loss, increased appetite and impaired motor skills." For his part, Holmes has been working to develop drugs that replicate the benefits but lack the negative side effects and don't create dependence. 

Researchers are making progress, however. Earlier this year, a study was published that suggested that using THC in conjunction with "extinction therapy," which Hamilton describes as being designed to "teach the brain to stop reacting to something that previously triggered a fearful response," is effective in combating PTSD. 

While twenty states have legalized medical marijuana, only six allow it to be used to treat PTSD. But that number is set to increase: thousands of veterans of the wars in Iraq and Afghanistan are currently pressing the government to allow them to use marijuana. One veteran with PTSD, Sgt. Ryan Begin, told NPR that, "Marijuana gives you that opportunity to think because it allows you to be more conscious of what's going around you. It just allows you that chance, that opportunity to breathe."

Whether or not researchers are able to turn marijuana into a wonder drug, its role in treating PTSD is sure to increase dramatically over the coming months and years.