The emergency contraceptive pill can be ineffective in overweight women, experts have warned.
The Faculty of Sexual and Reproductive Healthcare said the pill can fail for women with a body-mass index (BMI) of over 26 or in those who weigh 11 stone or more.
For safety 11 stone or a BMI of over 26 is the level we can say weight may create a risk
Dr Jane Dickson, FSRH
Newly published advice from the FSRH urges pharmacists and doctors to explain the risks to larger women, in whom the drug is diluted and broken down more quickly.
Dr Jane Dickson, vice president, said that although the 11 stone figure was based on research, it may not apply to all women.
“It may be 15 stone is the danger point for some women,” she said.
“But for safety 11 stone or a BMI of over 26 is the level we can say weight may create a risk.”
She said that larger women worried about pregnancy after contraceptive failure or unprotected sex could take two morning after tablets, or use an emergency coil.
“The coil’s effectiveness is not affected by a woman’s weight as it works differently to prevent fertilisation - it’s toxic to sperm and eggs and works locally,” said Dr Dickson.
The morning after pill disrupts fertilisation partly by delivering a higher dose of progestin, which is found in regular birth control pills.
The FSRH said both Levonelle and ellaOne, two of the most popular morning after pills, were thought to be affected by weight.
But HRA Pharma, which manufactures ellaOne, said the drug “continues to be the most effective oral option for most women at the standard dose regardless of their weight or BMI.”