Snapshots of two clean-cut teenagers, a boy and a girl, stare out from a newspaper advertisement for the vaccine Gardasil. “Boys can be affected by HPV disease too,” says the bold headline. The ad urges parents to protect “both your son and daughter.”

Until recently, Gardasil was a girls-only proposition. Approved for young women ages 9 to 26, the vaccine promised a great benefit: protection against four strains of sexually transmitted human papillomavirus (HPV), including two that can lead to cancer of the cervix, vagina and vulva.

Yet the vaccine has been a tough sell. It requires three shots, often painful, over the course of seven months. So far, only one in four teenage girls younger than 18 have completed all three shots.

Now, in the wake of new research suggesting that the vaccine protects against other cancers, Gardasil is increasingly marketed as an important vaccine for boys, too. The Food and Drug Administration has approved it for young men ages 9 to 26, expanding the list of indications just last December.