WASHINGTON — Drugmaker Merck says U.S. regulators have approved its vaccine Gardasil to prevent genital warts in boys, a new use for a product already given to women to prevent cervical cancer.

The Whitehouse Station, N.J.-based company says the Food and Drug Administration approved Gardasil for use in boys ages 9 through 26.

Gardasil already is approved in women to block four strains of the human papilloma virus that cause the majority of cervical cancers and genital warts. Merck has sold about 50 million doses worldwide, with more than $1.4 billion in revenue last year.

HPV infects about 6 million people in the U.S. each year, mainly through sexual contact. It usually causes no symptoms although rare cases can develop into warts and cancer in both men and women.