Low doses of marijuana may be clinically effective as an antidepressant because it increases serotonin levels, but researchers point out that heavier use of the drug can make depression worse and even cause psychosis, Fox News reported Oct. 25.

Animal studies conducted by researchers from McGill University, Le Centre de Recherche Fernand Seguin of Hôpital in Quebec, and l'Université de Montréal found "a double effect," according to researcher Gabriella Gobbi. "At low doses [marijuana] increases serotonin, but at higher doses the effect is devastating, completely reversed," she said.

Researchers added that controlling dosage is difficult when smoking raw marijuana.

The study was published in the Oct. 24, 2007 issue of the Journal of Neuroscience.