The Whitehouse Station, N.J.-based company said 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.

While the approval could theoretically double the market for Merck's vaccine, analysts do not expect Gardasil to be widely used in boys.

Genital warts caused by HPV usually clear up by themselves, and the cancers caused by the virus are extremely rare in men.

Friday's announcement came just hours after rival GlaxoSmithKline reported that the FDA approved its vaccine Cervarix for use in the U.S. The vaccine already was available in Europe and elsewhere.

Glaxo is likely to face an uphill battle to establish itself in the U.S., where Gardasil has dominated the market since 2006. Besides an established brand, Merck's vaccine defends against HPV types that cause both cancer and genital warts. Whereas Cervarix does not target genital warts.

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.

Gardasil became an early success story for Merck after its 2006 launch, achieving blockbuster sales that are rare for a vaccine.

But sales have been slowing amid questions about the longevity of the vaccine's effect and its price tag of nearly $400.

Merck has tracked HPV immunity out to five years in women, and just three years in boys and men. Public health advocates have argued that Merck should have been required to show evidence of Gardasil's long-term effectiveness