It is thought the drug is absorbed by fat, lowering the dose in the blood.
The study by the University of Edinburgh in 2011 investigated Norlevo’s active ingredient, levonorgestrel.
It indicated obese women taking the emergency contraceptive were four times more likely to become pregnant than women taking the drug who were a normal weight.
The report’s authors recommended other emergency contraceptives were used in overweight and obese women, such as devices that could be implanted into the womb.
Frederique Welgryn, HRA Pharma’s head of women’s health, said the results were “surprising” and had led to “a lot of discussions” about the effectiveness of levonorgestrel.
The labelling on Norlevo is expected to be changed in 2014.
The development has raised concerns about other morning-after pills.
The US regulator, the Food and Drug Administration, said it was “currently reviewing the available and related scientific information on this issue”.-after pill ‘fails in obese’.