The Plan B pill, one version of the morning-after pill, is available without a prescription, except for women 17 and younger.
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The survey, which is the first of its kind to look specifically at use of the pills, also found that most women aren't using them as a substitute for regular birth control. Of those who reported having used emergency contraception between 2006 and 2010, 59 percent said they had used it only one, and 24 percent twice. Only 17 percent said they had used it three or more times.
Use of emergency contraception, however, varies widely by age and other demographic distinctions, according to the study, which was part of the NCHS's National Survey of Family Growth.
Users of emergency contraception were most likely to be between age 20 and 24 (23 percent), never married (19 percent) and have at least some college education.
By contrast those least likely to have used emergency contraception were older. Only five percent of those ages 30-44 reported having taken the pills. Only 6 percent of current or formerly married women reported having used the products, and only 6 percent of those with less than a high school education said they had used it.
Those who did use emergency contraception, however, were about evenly divided between in their reasons, with about half reporting a need due to failure of another contraceptive method and half due to unprotected sex.
Meanwhile, in a separate study based on the same set of data, NCHS researchers found that "virtually all women of reproductive age in 2006 to 2010 who had ever had sexual intercourse have used at least one contraceptive method at some point in their lifetime."
That number — 99.1 percent of women ages 15-44 — includes methods such as natural family planning that involves periodic abstinence and withdrawal.
But the survey also found that 87.5 percent of women who had ever had heterosexual sex used what the researchers termed a "highly effective reversible method" of contraception, including the birth control pill, contraceptive patch, injectable drug such as Depo-Provera, or an intrauterine device.
And Catholic women reported using artificial birth control in smaller numbers than women of other religions, that use was still fairly widespread. According to the study, 89 percent of Catholic women reported having used a condom with a male partner, compared to 95-97 percent of Protestant women. Similarly, about 76 percent of Catholic women said they had used the birth control pill, compared to 86 percent of Protestant women.