A synthetic form of THC, the active ingredient in marijuana, acts as an antidepressant at low doses but can worsen psychiatric conditions at higher doses, according to a study in the Journal of Neuroscience.

The report was based on a study of rats. Sometimes, results seen in animal testing do not translate to people.

At the low doses, the authors said, THC acts like Prozac or Celexa to raise the level of serotonin in the brain. But beyond a certain point, the effect reversed.

"Low doses had a potent antidepressant effect, but when we increased the dose, the serotonin in the rats' brains actually dropped below the level of those in the control group. So we actually demonstrated a double effect," said lead author Gabriella Gobbi of McGill University.

Gobbi said she wanted to test pot's effect because of anecdotal evidence it eased depression.

"As a psychiatrist, I noticed that several of my patients suffering from depression used to smoke cannabis. And in the scientific literature, we had some evidence that people treated with cannabis for multiple sclerosis or AIDS showed a big improvement in mood disorders," she said.

But she said there are dangers with using pot directly as an antidepressant, especially because smoking it makes it hard to control dosage.