Image: HRA PharmaA French contraceptive maker says its morning-after pill is ineffective when taken by women who weigh more than 176 pounds.

The decision by HRA Pharma follows a publication of results of a study of levonorgestrel, one of the active ingredients in its Norlevo emergency contraceptive, says Frederique Welgryn, HRA Pharma's head of women's health.

Welgryn says that while the results of the study conducted by the Edinburgh Univ. in 2011 were "quite surprising," the last few years have seen "a lot of discussions" about contraceptives' efficacy in overweight or obese patients.

HRA Pharma Chief Executive Erin Gainer estimated that "millions" of women across Europe use emergency contraceptives identical to Norlevo. Gainer declined to give sales figures for Norlevo alone.

Anna Glasier, a lead researcher in the study, says that their research wasn't designed to look specifically at the effect of weight on emergency contraception. She says their study only included about 1,700 women.

"It is not my place to comment as to whether the company's decision to change advice is premature," she writes in an email.

She also notes another previous analysis that found there was no solid evidence to show that hormonal contraceptives were less effective in overweight women, but the quality of the studies was low.

Welgryn says a dose of HRA's drug contains 1.5 mg of levonorgestrel, identical to that found in Plan B One-Step, manufactured by Teva Pharmaceuticals, use identical ingredients.

Denise Bradley, a spokeswoman for Teva Pharmaceuticals, declined to comment.

HRA began the process of consulting with French regulators about changing the pill's labeling in 2012 and it has taken until now for Europe's drug regulators to approve the change, Welgryn says.

The new warning also says that the drug's efficacy is reduced in women who weigh more than 165 pounds, and it is ineffective in women who weigh more than 176 pounds.

Diana Mansour, a spokeswoman for Britain's Faculty of Sexual and Reproductive Healthcare, says in overweight and obese women the drug probably gets absorbed into their fat more quickly and doesn't have enough time to work in their bodies.

"It has less of a chance to delay ovulation and there are lower levels (in the blood) of the drug to have an effect," she says. Mansour says there wasn't enough information to know if simply giving obese women a higher dose of the drug would solve the problem.

In the U.K., doctors haven't been given any advice to stop giving overweight or obese women Levonnelle, which contains the same active ingredient as Norlevo.

Lynn Hearton, clinical lead for the FPA sexual health charity, says any overweight or obese women concerned about emergency contraception should speak to a medical professional.

"It is particularly important we don't put some women off taking emergency contraception because they think it won't work anyway," she says.

She says women should consider using an IUD, a small plastic birth control device which can be inserted in the womb up to five days after unprotected sex.

HRA markets Norlevo in about 50 countries worldwide. It began marketing the pill in France in 1999.

It will begin printing the warning on packaging in the first half of 2014.