VANCOUVER - Commonly prescribed anti-depressants may do more harm than good to patients and people need to be much more cautious about the widespread use of these drugs, a Canadian study suggests.

The study, conducted by researchers from McMaster University and published this week in online journal Frontiers in Psychology, challenged the belief that anti-depressants are safe and effective.

After examining previous studies into the effects of anti-depressants, the researchers found that elderly anti-depressant users are more likely to die than non-users, which indicates that the overall effect of these drugs on the body is more harmful than beneficial.

Most anti-depressants are designed to relieve the symptoms of depression by increasing the levels of serotonin in the brain, where it regulates mood, according to the study.

The vast majority of serotonin that the body produces, though, is used for other purposes, including digestion, forming blood clots at wound sites, reproduction and development, the study said.

The researchers found that anti-depressants have negative health effects on all processes normally regulated by serotonin. The risks include developmental problems in infants, digestive problems and abnormal bleeding and stroke in the elderly, according to the study.

"Serotonin is an ancient chemical. It's intimately regulating many different processes, and when you interfere with these things you can expect, from an evolutionary perspective, that it's going to cause some harm," says Paul Andrews, the lead author of the study.

In previous research, Andrews and his colleagues had questioned the effectiveness of anti-depressants even for their prescribed function, finding that patients were more likely to suffer relapse after going off their medications as their brains worked to re-establish equilibrium.

With even the intended function of anti-depressants in question, Andrews says it is important to look critically at their continuing use as millions of people are prescribed anti-depressants every year.