Cannabis: Potent Anti-Depressive In Low Doses; Worsens Depression at High Doses

http://www.sciencedaily.com/releases/2007/10/071023183937.htm

A new neurobiological study has found that a synthetic form of THC, the active ingredient in cannabis, is an effective anti-depressant at low doses. However, at higher doses, the effect reverses itself and can actually worsen depression and other psychiatric conditions like psychosis.