The French manufacturer of the morning-after pill, HRS Pharma, said that their product does not prevent pregnancies in women who weigh over 80kgs.
The company made the revelation after findings of a study of levonorgestrel, a key ingredient in its Norlevo emergency contraceptive, were published, the Guardian reported.
However, Anna Glasier, a lead author of the study, said that the research wasn't particularly aimed at studying the effect of weight on emergency contraception and merely included about 1,700 women.
But, the French contraceptive maker has decided to issue a warning related to the drug's inefficiency in overweight women along with the packaging.
A spokeswoman for UK's Faculty of Sexual and Reproductive Healthcare said that in overweight and obese ladies the drug probably gets absorbed into their fat more quickly and doesn't have enough time to delay ovulation.