Nov 25 (Reuters) - The U.S. Food and Drug Administration said it was looking into whether morning-after emergency contraceptive pills fail to work in women over a certain weight and will decide whether to add a warning to the drug's label.

The agency was reacting to reports that European health regulators had ordered a label change for the emergency contraceptive product sold there by French drugmaker HRA Pharma under the brand name Norlevo, requiring it to include the drug's diminishing effectiveness based on weight.

The FDA is "currently reviewing the available and related scientific information on this issue, including the publication upon which the Norlevo labeling change was based," FDA spokeswoman Erica Jefferson said on Monday via an email. "The agency will then determine what, if any, labeling changes to approved emergency contraceptives are warranted."

HRA scientists discovered that its Norlevo began losing effectiveness in women who weigh about 165 pounds (75 kilograms) and was not at all effective in women weighing over 176 pounds (80 kg), according to reports on CNN and elsewhere.

HRA was conducting research on another topic related to Norlevo when scientists realized there was "a clear impact of weight" on the drug's effectiveness, HRA Chief Executive Erin Gainer said, according to the CNN report.

The Norlevo label is expected to be changed early in 2014, reports said.

Norlevo is similar to morning-after contraceptives sold in the United States under the brand name Plan B One Step and in generic versions under other names. Plan B is sold by Israel-based Teva Pharmaceutical Industries Ltd.

Teva did not immediately respond to calls or emails seeking comment. HRA Pharma also could not immediately be reached for comment.

The FDA in June expanded the availability of emergency contraceptives, which are taken following sex to prevent pregnancy, to comply with a U.S. court order.

The pills are now available in the United States to all women of child-bearing age as an over-the-counter product without age or point-of-sale restrictions.