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 (CDC) 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.
The CDC's findings come amid a renewed fight over birth control access as religious groups push back against President Barack Obama's 2010 health care law, which includes a provision requiring health insurance coverage of contraception.
Many conservative groups, including Catholic institutions that oppose use of artificial birth control - especially morning-after pills, are challenging the rule, saying its religious exemption is too narrow.
A separate CDC report, also released on Thursday, found that 99 percent of sexually active women of reproductive age have used some form of contraception, with the use of condoms and longer-term, non-pill methods on the rise.
Emergency contraception has been available by prescription in the United States since 1999. One version, 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 U.S. Food and Drug Administration (FDA) approved sales of Plan B to adult women without a prescription in 2006 after years of contentious debate. It later allowed sales to 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 with no age restrictions after U.S. Health Secretary Kathleen Sebelius in 2011 took an unprecedented step of intervening in an FDA decision and rejecting the latest petition to loosen sales.
"This data shows the importance of expanding access to emergency contraception to all women of reproductive age," said Deborah Nucatola, senior director of medical services at Planned Parenthood Federation of America.
Critics, who liken the drug to abortion, lamented the findings.
The rise in emergency contraception use "is the sad result of deceptive labeling," Anna Franzonello, an attorney for Americans United for Life.
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.
CONTRACEPTION AND COSTS
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.
Overall, the number of women using regular birth control pills has remained flat over time while the use of injections, patches and intrauterine devices has grown, CDC reported in separate findings. The number of women whose partners have used condoms also rose, data 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 the Guttmacher Institute, which also tracks contraception 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 at higher risk for unintended pregnancies, said Lawrence Finer of the reproductive research group.
That could change in the wake of Obama's healthcare overhaul, added Finer, who oversees the institute's domestic research.
The law aims to extend health insurance to more people, including lower income Americans, and requires insurers to cover prescription birth control without a co-payment.