updated 1/28/2010 7:05:11 PM ET 2010-01-29T00:05:11

A new type of morning-after pill is more effective than the most widely used drug at preventing pregnancies in women who had unprotected sex and also works longer, for up to five days, a new study says.

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The report was published Friday in the British medical journal, Lancet.

Levonorgestrel, the most widely used emergency contraceptive pill, is only effective if women take it within three days of having sex. It is sold under various brand names including Levonelle and Plan B, and is available in more than 140 countries, including the United States, Canada and many countries in western Europe. In nearly 50 of those countries women can get it without a prescription.

International researchers compared Plan B to the new drug ulipristal acetate, sold as ellaOne in Europe only with a doctor's prescription. The drug is not legally on the market elsewhere.

Experts tracked nearly 1,700 women aged 16 to 36 who received emergency contraception within three to five days of having unprotected sex. About half got Plan B while the rest got ellaOne.

In the group that got Plan B, there were 22 pregnancies. In those that got ellaOne, there were 15. In both groups, the most frequently reported side effect was a headache. The research was paid for ellaOne's maker, HRA Pharma, which helped design the study.

When the researchers pooled their results with a previous study comparing the two morning-after pills, they found women who took ellaOne within five days after sex almost halved their chances of becoming pregnant compared to women who took Plan B.

Women who took ellaOne had a 1.8 percent chance of becoming pregnant, while women who took Plan B had a 2.6 percent chance.

Last May, the European drug regulator approved ellaOne. HRA Pharmacy has refused to say where else the pill is being submitted for approval.

Compared with Plan B, which becomes less effective over time, ellaOne appears to work consistently well over five days in women who have unprotected sex.

Health officials, however, warned that this should not give women a false sense of security.

"The message has to be always that women should act as soon as possible," said Tony Kerridge, a spokesman for Marie Stopes International, a nonprofit sexual health organization in Britain not linked to the study.

"You may think you have a window of opportunity, but as soon as you can, go somewhere and get it sorted," Kerridge said.

Plan B contains synthetic progesterone and mimics how the natural hormone works, interfering with ovulation in the early stages of the egg's development. In contrast, ellaOne delays ovulation until the egg is released from the ovary.

That difference may explain why ellaOne works for two more days than Plan B, said Dr. Anna Glasier of the Family Planning and Well Woman Services at Dean Terrace Centre in Edinburgh, Scotland, who led the Lancet study.

Glasier said more safety data is needed before ellaOne could be recommended for over-the-counter use.

"This is still a new drug, and it has to be around for a couple of years without seeing any unexpected adverse events before anyone would contemplate making it available without prescription," Glasier said.

She estimated that ellaOne costs about three times the price of Plan B.

Copyright 2010 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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