Previous research has indicated that marijuana can exacerbate mood disorders such as anxiety and depression, according to Dr. Elie Aoun, assistant professor of clinical psychiatry at Columbia University College of Physicians and Surgeons. However, it is important to note that this association does not imply that using marijuana causes or will cause mental illness. Rather, it may be the case that people who use marijuana are more likely to also suffer from mental illness. There is some evidence to support this connection. For example, studies have shown that individuals who use marijuana daily for several years tend to report lower rates of happiness compared to those who do not use the drug regularly. Also, people who abuse marijuana are about five times more likely than the general population to experience symptoms of depression. Finally, research has shown that patients who use marijuana to treat pain tend to experience less relief from their pain and are more likely to want to sleep even when they don't need to sleep than before they started using the drug. It is possible that long-term exposure to THC, the main psychoactive component in marijuana, may be responsible for altering brain chemistry in ways that lead to changes in behavior and emotion. Studies have shown that when humans use marijuana regularly over time, certain cells in their brains begin to produce more of the receptor type one found in depressed people, says Dr. Aoun.