Contraceptive company plans to put warning on packets after study shows emergency pill is ineffective in women over 12st 7lb

A French contraceptive manufacturer said on Tuesday that its morning-after pill is ineffective when taken by women who weigh more than 80kg (12st 7lb). The announcement by HRA Pharma follows publication of results of a study of levonorgestrel, one of the active ingredients in its Norlevo emergency contraceptive, said Frederique Welgryn, HRA Pharma's head of women's health.

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

Erin Gainer, HRA Pharma's chief executive, 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, said that the research wasn't designed to look specifically at the effect of weight on emergency contraception. She said the study included only about 1,700 women.

"It is not my place to comment as to whether the company's decision to change advice is premature," she said in an email. She also noted 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 said a dose of HRA's drug contains 1.5mg of levonorgestrel, identical to that found in Plan B One-Step, manufactured by Teva Pharmaceuticals, which uses identical ingredients. Denise Bradley, a spokeswoman for Teva Pharmaceuticals, declined to comment.

HRA began consulting French regulators about changing the pill's labelling in 2012. It has taken until now for Europe's drug regulators to approve the change, Welgryn said. The new warning also says that the drug's efficacy is reduced in women who weigh more than 75kg (11st 12lb), and it is ineffective in women who weigh more than 80kg.

Dr Diana Mansour, a spokeswoman for Britain's Faculty of Sexual and Reproductive Healthcare, said that 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 said. Mansour said there wasn't enough information to know if simply giving obese women a higher dose of the drug would solve the problem.

In the UK, 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, said 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 said.

She said 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 Pharma 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.