The US Food and Drug Administration is looking into whether morning-after contraceptive pills fail to work in women over a certain weight.
It will decide later 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 company HRA Pharma under the brand name Norlevo, requiring it to include the drug's diminishing effectiveness based on weight.
HRA scientists discovered that Norlevo began losing effectiveness in women who weighed about 75kg and was not at all effective in women heavier than 80kg .
