Cannabis shows anti-depression benefits, but too much has reverse effect

October 22, 2007

, Canadian Press

Cannabis in small doses has some beneficial effects for curbing depression, but too much of the drug can cause the polar opposite effect, a new study suggests.

The neurobiological study conducted jointly by McGill University and a research institute affiliated with the Universite de Montreal indicates cannabis in low doses increased serotonin levels in the brains of laboratory rats.

The depletion of serotonin, a neurotransmitter that controls moods, leads to depression. Anti-depression drugs work by increasing the serotonin in the brain.

However, when the doses were increased on the rats, the effect was described as completely reversed, even devastating. Serotonin levels dropped significantly.

"We know from anecdotal evidence that (human) smokers of marijuana experience good effects ... but they also experience unwanted effects," said McGill PhD student Francis Bambico, who authored the study, which is to be published Wednesday in the Journal of Neuroscience.

"Some smokers experience anxiety, agitation. But at very high doses and in certain circumstances, it leads into depression-enhancing effects, particularly if you take in a lot of cannabis at very high doses."

Humans using cannabis as an anti-depressant is problematic since it's hard to control the dosage when it is smoked in the form of marijuana joints, says lead researcher Dr. Gabriella Gobbi.

"Our research shows that excessive cannabis use in people with depression poses a high risk of psychosis," Gobbi said in a telephone interview from Italy.

A psychiatrist by trade, Gobbi says she was prompted to conduct the research after noticing several of her patients suffering from depression used to be cannabis smokers.

Rats were injected with a synthesized cannabinoid which worked as a potent anti-depressant at low levels, varying from 0.05 to 0.2 milligrams per kilogram.

But higher doses showed the opposite effects that "totally undid" the anti-depression effects, Bambico said.

Bambico said researchers are focusing their research on a new class of drugs which enhance the effects of the brain's natural endo-cannabinoids - natural substances in the brain that are released during high stress or pain, without the anti-depressive side-effects.

Cannabis has a chemical similarity to endo-cannabinoids.

"It's entirely possible to produce drugs that enhance endo-cannabinoids for the treatment of pain, anxiety and depression," Gobbi said.

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