Certain types of anti-depressants can increase the risk of breast cancer according to a new study.

The researchers, from Montreal's Sir Mortimer B. Davis-Jewish General Hospital, recommend doctors stop prescribing certain drugs.

Their concerns revolve around some tricyclic anti-depressants specifically:

  • Amoxapine (sold as "Asendin")
  • Clomipramine ("Anafranil")
  • Despramine ("Norpramin")
  • Trimipramine ("Surmontil" and "Rhotrimine")
  • Paroxetine ("Paxil")

Researchers conducted an investigation among women 35 years or older enrolled in the Saskatchewan Prescription Drug Plan from 1981 to 1995. The women were taking 10 different kinds of anti-depressants.

They found those taking the six anti-depressants had their risk of breast cancer double. Five are known to be "genotoxic," which means lab experiments have shown they can damage DNA.

Paxil is in another class of drugs known as SSRIs (selective seratonin reuptake inhibitor). Others include Prozac and Zoloft. Although research on Paxil is relatively new, studies have shown it too can have the same effect on cancer risk.

Earlier research done by Michelle Cotterchio of Cancer Care Ontario found that women who took Paxil saw their risk of breast cancer increase by seven times.

The breast cancer for genotoxic drugs did not show up until 11 to 15 years after the initial prescription.

"The 10 year delay...suggested to us that the drugs might be acting as tumour initiators," said Dr. Colin Sharpe, the lead researcher, in a news release.

Sharpe cautions that patients need not panic and throw away their medication. Breast cancer organizations say more studies need to be conducted on this link.

"The risks and benefits of anti-depressant medication have to be balanced and should always be discussed with the woman's physician," said Dr. Marilyn Schneider of the Canadian Breast Cancer Research Initiative, which funded the research.

More than one million Canadians are believed to be taking anti-depressants and about a quarter of them are taking tricyclics. Researchers say not all of them are at risk.

The study is published in the British Journal of Cancer.