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: