One-third of American teens using the birth control pill do so for reasons other than contraception, according to a new report.

More than 11 million women between the ages of 15 and 44 are currently on the pill. About 14 percent of them take it solely for non-contraceptive reasons, says study author Rachel Jones, a senior research associate at Guttmacher Institute. About 9 percent continue to take the pill despite not having sex in three months or more.

"They do have sex unexpectedly, so they're prepared for it," she said.

But as many as a third of teenagers use it for other reasons, the report concludes, such as reducing cramps, migraines and pain related to menstruation, having more regular periods, and as a treatment for acne.

The study was based on in-person interviews of over 7,000 women, ages 15 to 44, as a part of the Centers for Disease Control and Prevention's National Survey of Family Growth for 2006 through 2008.