A noted prostate cancer researcher is not pleased with a warning issued this week by Health Canada about two popular medications used to treat enlarged prostates.

On Monday, Health Canada warned that finasteride – sold generically as well as under the name Proscar -- and dutasteride – sold as Avodart and Jalyn -- can increase the risk for high-grade prostate cancer.

It noted that two key studies had shown that some men taking the drugs have been diagnosed with high-grade prostate cancer, an aggressive form of the disease which grows and spreads more quickly than low-grade cancers.

The agency said the increased risk seen with finasteride and dutasteride drugs was considered "very small," but it said new warnings would be added to the labels for the drugs.

The warning was based on Health Canada's review of the Prostate Cancer Prevention Trial (PCPT), and the Reduction by Dutasteride of Prostate Cancer Events (REDUCE) trial.

(A lower dosage of finasteride to treat male baldness, sold under the same Propecia, was also included in the warning. While this strength of the drug was not included in the trials that were reviewed, Health Canada said that a potential risk from this strength of drug "has not been ruled out.")

Dr. Neil Fleshner, a urologic oncologist at Princess Margaret Hospital in Toronto, disagrees with the Health Canada warning and is concerned it will needlessly worry men who could actually benefit from the drugs.

Fleshner has been researching the use of dutasteride as a way to help slow the growth of early stage prostate cancer. His research has found that giving the drug to men with low-risk prostate cancer actually helps to slow the progression of prostate cancer.

He believes the Health Canada warning is "short sighted."

"Dutasteride and finasteride… lower the risk of prostate cancer. In well-conducted, large scale trials, both drugs can reduce a man's chance of getting prostate cancer by about 25 per cent," he told CTV News in an email.

Fleshner concedes that some cancer prevention studies have noted "a very small" number of men treated with these drugs seem to develop more aggressive forms of prostate cancer -- around one in 200.

But he does not believe the drugs cause the cancers. Instead, he suspects they might actually make the cancer more easily detectable.

"We and others have shown in research studies that by shrinking the prostate (which these drugs do), one can unmask the presence of pre-existing forms of more aggressive cancers. In other words, these drugs don't cause aggressive cancers; they make them easier to find. This is arguably a benefit of the drug," Fleshner writes.

He says there hasn't been enough research yet to prove whether his suspicion is true.

"But we should not negate the benefits of these drugs for a rare adverse event that is almost certainly not caused by the drug but merely an artifact of its use," he says.

He believes that men should not stop the use of these drugs for fear of high grade cancers. Instead, they should book an appointment with their doctors to talk about the risks and benefits of the drugs.

With files from CTV's medical specialist Avis Favaro