Last November, HRA Pharma changed the labels on its morning-after pill Norlevo, marketed in 50 countries, saying it was ineffective in women over 80 kilograms after a study of levonorgestrel, one of the drug's active ingredients.
Norlevo is not sold in the US but another drug with the same main ingredient, Plan B One-Step, is widely available.
The Food and Drug Administration said last year it was reviewing the issue to see if any label changes were needed.
The morning-after pill contains a higher dose of the hormone in regular birth control pills. Taking it within 72 hours of sexual intercourse can cut the chances of pregnancy by up to 89 per cent but it works best within the first 24 hours. If a woman is already pregnant, the pill, which prevents ovulation or fertilization, has no effect.