More US women are taking the "morning-after" pill, but generally just once, according to the government's first report on how the emergency contraception drug has been used since regulators eased access to it in 2006.
About 11 percent of sexually active women, or 5.8 million, used the pill between 2006 and 2010, compared to about 4 percent in 2002, the Centers for Disease Control and Prevention said in its report released on Thursday.
Among those who used the pill during those four years, 59 percent said they took it just once, while 24 percent said they used it twice, the report said. Seventeen percent said they used it three times or more.
Emergency contraception has been available by prescription in the United States since 1999. One version of the morning-after pill, known as Plan B, has stirred the most political controversy.
Plan B, much like regular birth control, stops pregnancy by blocking the release of a woman's egg, or it may prevent fertilization or implantation in the uterus. But it must be taken within days after intercourse to work.
The US Food and Drug Administration approved sales of Plan B to adult women without a prescription in 2006 after years of contentious debate. It later loosened the restriction to include 17-year-olds.
Women's health groups lauded the move as a way to prevent unwanted pregnancies. But conservatives warned it could lead to promiscuity, especially among youth, and more sexual assaults.
Amy Allina of the National Women's Health Network said CDC's findings show morning-after pills are not replacing conventional birth control methods for most women, although "there are some for whom it's clearly not a one-time thing."
Activists are still pressing for over-the-counter access and no age restrictions.
The pill is sold by Teva Pharmaceutical Industries Ltd as Plan B. It also is available as a generic. In 2010 the FDA approved another emergency contraceptive called ella, a prescription drug now owned by Actavis Inc.
CDC's findings showed the reasons for emergency contraception use varied depending on race and education levels.
Hispanics and blacks were more likely than whites to report using the drug after unprotected sex. More white women said they used it because they were worried their other birth control method had failed, CDC said.
Those with at least some college education were more likely to use the pill than those with a high school education or less, according to the report, which is based on data from the CDC's National Survey of Family Growth.
"The women who are less likely to have access to healthcare are more likely to say 'I didn't use another method, and I turned to emergency contraception to protect myself,'" said Allina.
Some women may choose to use it occasionally if they cannot afford other methods, she added.
In a separate report on Thursday, CDC looked at overall contraceptive use and found that while the number of women using regular birth control pills has remained flat over time, the use of injections, patches and intrauterine devices has grown.
The number of women whose partners have used condoms also rose, the findings showed.
That trend may reflect increased wariness among Americans to have children amid the 2007-2009 economic recession, the effects of which are still being felt by many, according to researchers at the Guttmacher Institute, which also tracks birth control use.
"At the same time, it can make it harder for people to have access to birth control because of costs," especially for disadvantaged women who face higher rates of unintended pregnancies, said Lawrence Finer, head of domestic research for the reproductive research group.
That situation could change in the wake of the 2010 healthcare overhaul that required health insurers to begin covering birth control last year, although the law faces legal challenges.
Religious groups, particularly Catholics, charge that the provision violates their belief against artificial birth control and are fighting to block it.