PARIS — A French contraceptive maker said Tuesday that its morning-after pill does not work when taken by women who weigh more than 176 pounds and plans to change its labels to warn patients.
The decision to change its labels by HRA Pharma is based on a previous study of levonorgestrel, one of the active ingredients in its Norlevo emergency contraceptive, said Frederique Welgryn, HRA Pharma’s head of women’s health.
Some doctors said that heavy women should not assume emergency contraception will not work and that they should talk to a health professional or consider alternatives like a copper IUD, a birth control device that can be fitted on the uterus.
Norlevo is not sold in the United States but another drug with the same ingredient is.
Welgryn said a dose of HRA’s drug contains 1.5 milligrams of levonorgestrel, identical to that found in Plan B One-Step, manufactured by Teva Pharmaceuticals in the United States.
The Food and Drug Administration will determine ‘‘what, if any, labeling changes to approved emergency contraceptives are warranted,” said spokeswoman Erica Jefferson.
Welgryn said that while the results of the study conducted by the University of Edinburgh in 2011 were surprising, the last few years have seen much discussion about contraceptives’ efficacy in overweight or obese patients.
HRA Pharma’s chief executive, Erin Gainer, estimated that millions of women across Europe use emergency contraceptives identical to Norlevo.
Anna Glasier, a lead researcher in the 2011 study, said their research was not designed to look specifically at the effect of weight on emergency contraception. She said their study 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 said by e-mail.
She also noted a 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.
The morning-after pill contains a higher dose of the hormone used in regular birth control pills.
Taking it within 72 hours of rape, condom failure, or forgetting regular contraception can cut the chances of pregnancy by up to 89 percent, but it works best within the first 24 hours. If a woman is already pregnant, the pill, which prevents ovulation or fertilization of an egg, has no effect.
The warning also says that the drug’s efficacy is reduced in women who weigh more than 165 pounds, and is ineffective in women who weigh more than about 176 pounds.