WASHINGTON – A panel of federal health experts on Wednesday rejected the use of two drugs from Merck and GlaxoSmithKline to prevent prostate cancer, saying the drugs could actually raise the risk of the most serious types of tumors.

The Food and Drug Administration panel of cancer experts voted 17-0 with one abstention that the risks of Merck’s Proscar outweighed its benefits. In a similar vote, the panel voted 14-2 with two abstentions against GlaxoSmithKline PLC’s Avodart. Both drugs are already approved to treat enlarged prostate.

“In a setting like this, the onus is on the drug to be completely safe, and to show benefit, and I don’t think it reached that level,” said panel Chairman Dr. Wyndham Wilson, of the National Cancer Institute.

The FDA is not required to follow the panel’s advice but often does.

Studies by both companies showed their drugs decreased incidence of low-grade prostate tumors by nearly 25 percent.

However, panelists voiced concern that a small number of men taking the drugs actually developed more aggressive tumors, compared with patients taking a dummy pill. Panelists said the risk of increasing deadly tumors outweighed the benefit of reducing tumors that are seldom fatal.