Some Morning After Pills Found Ineffective for Many Women

An emergency contraceptive manufactured in Europe will come with a new label in 2014, warning that the pill may not be effective for women over a certain weight.

The same may be true for emergency contraceptives in the United States.

French manufacturer HRA Pharma was conducting research on another topic related to its emergency contraceptive Norlevo when scientists realized there was "a clear impact of weight" on the drug's effectiveness, CEO Erin Gainer said.

HRA Pharma scientists discovered Norlevo began losing its effectiveness when women reach about 75 kilograms, or 165 pounds, Gainer said, and showed an "absence of effectiveness" at about 80 kilograms, or about 176 pounds.

"We felt it was our ethical duty ... to report those results to our health authorities here in Europe," Gainer said.

"One of the prevailing theory’s is that for the drug to work it, has to reach a certain concentration in the bloodstream. And for larger people it may require a higher dose. They're larger, they may need more of the medicine. But this drug is given in one dosage,"  CNN's Elizabeth Cohen reports.

Norlevo is identical to the emergency contraceptive Plan B One-Step, said Kelly Cleland, a public health expert at Princeton University. Both are LNG ECs, or emergency contraceptives that include levonorgestrel, a synthetic version of the hormone progestogen. Emergency contraceptives are often called the morning-after pill because they work by interfering with ovulation, preventing the fertilization of an egg.

Teva Pharmaceuticals, which manufactures Plan B, did not respond to CNN's request for comment.

In Europe, they're going to put warnings on the label about to do if you weigh more than 176 pounds, Cohen says. “And we’re told by the FDA, they are looking into whether they should do the same thing in this country, too.”