A class of drugs used to treat enlarged prostates in men may increase the risk of being diagnosed with a more serious form of prostate cancer, the U.S. Food and Drug Administration (FDA) said.

Drugs in the class include Avodart and Jalyn, marketed by GlaxoSmithKline, and Proscar, marketed by Merck, as well as another Merck drug Propecia, which is approved to treat hair loss in men.

Propecia contains a lower dose of the same active ingredient that is in Proscar.
In a statement posted on FDA's website, the agency said the risk of developing high-grade prostate cancer, or a more aggressive form of the disease, "appears to be very low." The product labels of the drugs have been revised to warn doctors and patients of the risk of developing high-grade cancer and to state that they are not approved to prevent prostate cancer.

The agency noted the benefits of the products outweigh the risks.

GlaxoSmithKline had been seeking FDA approval to market Avodart as a prostate cancer prevention treatment on the basis of a clinical study that involved 8,000 men, which showed the product cut the risk of prostate cancer among those treated with the drug for four years compared to those taking a placebo. The FDA declined to approve Avodart for that use in January.

The FDA said a decline in the risk of developing prostate cancer in the Avodart study, and another study involving Proscar, was driven by a decline in low-grade, or low-risk, cancers that are slow growing and typically are not life threatening.

Both studies showed a small increase in high-grade prostate cancer among men being treated with Avodart or Proscar compared to those not receiving the drug.

In a statement, Merck said it stands behind the safety of Proscar and is "disappointed" that the updated drug label incorporates "only select risk-related data" rather than a fuller discussion of the benefit-risk profile. Proscar is also available as a generic drug.

GlaxoSmithKline did not immediately return a request for comment.