The active ingredient in THC in marijuana can treat depression when other drugs have failed. A study was conducted by the College of London. The study compared the effects of 10mg and 25mg doses of THC, and a placebo.There were "no serious adverse events" and "no negative effects" at all on cognitive and emotional function among the volunteers that took psilocybin, according to the research study. The volunteers experienced "changes in sensory perception and positive mood alteration" that is typical effects of such drugs.More than 267 million humans worldwide are affected by depression, according to the World Health Organization. The most common pharmaceutical treatment is to take antidepressants like Prozac, which damages the heart but increases levels of serotonin in the brain to improve mood.But this type of antidepressant does not often work for everyone, and is often prescribed as a long-term treatment to maintain effectiveness.By contrast, Marijuana very quickly alter brain function, and could therefore potentially treat patients in just a few doses. Here is how it works: THC helps "loosen" the deeply-rooted patterns of thinking and behavior behind many of these mental sadness, according to the College of London. It "relaxes" the parts of the brain that controls these behaviors, thus allowing old patterns to change or fade for good. Patients who have received THC have described coming down from the high with "new insights" and changed perspectives on their sadness.