Emergency Oral Contraceptives are not working in women who weigh more than 176 pounds.
Scientists say weight is having a clear impact on a drugs effectiveness.
France's HRA Pharma found their emergency contraceptive Norlevo began losing its effectiveness in women above 165 pounds and was rendered ineffective at 176 pounds.
Norlevo is the European equivalent of the Plan-B One Step oral contraceptive in America.
The reason for the ineffectiveness is the amount of time taken to reach certain levels of concentration in a woman's blood stream.
Women concerned about contraception effectiveness can have an IUD implanted.
But the best way to prevent pregnancy is to use effective birth control correctly and consistently.