Some antidepressants in the same class as Prozac significantly increase the chance of stomach bleeding, say experts

Particularly at risk are the elderly and those who already had a history of upper gastrointestinal bleeding.

There has already been one piece of research which suggests the link between bleeding and "selective serotonin re-uptake inhibitors", or SSRIs.

However, the latest study from the Ottawa Health Research Institute in Canada, published in the BMJ on Friday, is the most powerful evidence yet.

Millions of people worldwide are prescribed either Prozac or other types of SSRI, which appear to work by increasing the amount of a brain chemical called seratonin available to have an effect on the organ.

If research can pinpoint who will do well and who won't it would be a very good idea

Dr David Healy

The researchers looked at the health records of well over 300,000 elderly people taking antidepressants.

They were placed in groups depending on the relative strength of action of the drug they were taking.

The group taking the most powerful SSRI had an increased bleeding risk of approximately 10% compared to the group taking the least powerful SSRIs.

The change was even more pronounced in those aged over 80.

Medical history

The authors concluded that it was important that doctors looked at the medical history of patients before deciding which strength of SSRI to prescribe.

The earlier Spanish study found SSRI users were three times more likely than average to develop gastrointestinal bleeding.

The risk was roughly equivalent to long-term treatment with anti-inflammatory drugs such as ibuprofen.

This is not the first time that the class of drugs which includes Prozac has been linked to a serious side-effect.

Some evidence suggests that it can produce suicidal feelings in a few patients in the first days they are taking the drug.

Dr David Healy, a Wales-based researcher who has conducted his own studies into SSRI, told BBC News Online: "I have to hope that with the right kind of warnings, that these drugs are worthwhile for patients.

"Some people will certainly respond to them very well, while others will not.

"If research can pinpoint who will do well and who won't it would be a very good idea."