Federal health officials issued a warning Thursday that drugs used to treat the prostate gland and baldness may increase the risk for the most serious form of prostate cancer.

The Food and Drug Administration issued a safety alert that a group of drugs called 5-alpha reductase inhibitors -- which are sold under the names Avodart, Proscar, Jalyn and Propecia -- may boost the chances of developing “high-grade” prostate cancer.

The alert is based on the agency’s review of two large studies, which showed that the drugs reduced the overall risk of getting prostate cancer but increased the chances of developing high-grade tumors.

The drugs are used to shrink enlarged prostate glands, a condition known as benign prostatic hyperplasia, or BPH. Proscar and Avodart are also approved to reduce the risk of urinary retention or surgery related to an enlarged prostate. Propecia is approved to treat “male pattern” hair loss.

Merck manufactures Proscar and Propecia. GlaxoSmithKline manufactures Jalyn and Avodart. The drugs are also known as dutasteride and finasteride.

Earlier this year the FDA refused a request from both companies to approve the drugs for use in preventing prostate cancer, citing the increased risk for the more serious tumors.