Use of the morning-after pill in the US is on the rise, a trend driven largely by wider use among younger women, a major government study has found.
Some 11% of sexually active women had used the morning-after pill, up from 4% in 2002, according to the US Centers for Disease Control and Prevention.
One in four women aged 20-24 had used the drug, compared with one in nine women overall.
The CDC surveyed more than 12,000 females aged 15-44 from 2006-10.
The research showed that white and better educated women used the drug the most, usually due to worries that another birth control method had failed.
Hispanic and black women were more likely than whites to report using the drug after unprotected sex.
Overall, about one in five women who had never been married had taken a morning-after pill, compared with just one in 20 married women.
Of the women who used the pill, 59% said they had done it only once, 24% said twice, and 17% said three or more times.
It was the first government report on the emergency contraceptive since regulators eased access to it seven years ago.
Emergency contraception has been available by prescription in the US since 1999, before regulators approved over-the-counter sales of the drug in 2006.
Also on Thursday, the CDC released a separate study on overall contraceptive use. It found that while the number of women using regular birth control pills had remained steady, the use of patches, intrauterine devices and injections had grown.