The drug finasteride, sold under the brand names Proscar and Propecia and their generic equivalents, is prescribed to treat both enlarged prostate and male-pattern baldness.
Dutasteride, available under the brand names Avodart and Jalyn, is also used to treat an enlarged prostate, a condition also known as benign prostatic hyperplasia.
Health Canada says finasteride and dutasteride may be associated with a higher risk of developing high-grade prostate cancer, an aggressive type of the disease that can grow and spread quickly.
The department says high-grade prostate cancer is rare, and the increased risk seen with finasteride and dutasteride is still considered very small.
The new safety information is based on a Health Canada review of two large international clinical trials that showed that long-term daily use of finasteride and dutasteride in men 50 and older was linked to a small increased risk of high-grade prostate cancer.
In the studies, long-term use was described as four years or more.
The risk was shown in the five milligram dosage of finasteride (sold as Proscar). The one-mg strength of the drug (sold as Propecia) was not included in these trials, but a potential risk has not been ruled out, Health Canada says.
The trials were conducted to see if the two drugs could be used to prevent prostate cancer. Both studies found the possible benefit of preventing low-grade prostate cancer was small compared to the risk of developing the high-grade form of the disease.
Finasteride and dutasteride are not approved for the prevention of prostate cancer in Canada.
Patients should discuss questions or concerns about treatment with finasteride or dutasteride with their doctor, and should not stop taking their medication unless advised to do so.