Britain's most popular morning-after pill is to be reviewed by the drugs watchdog, after a manufacturer admitted that it would not work for the millions of women who weigh more than 121/2 stone.
Levonelle also becomes less effective at preventing pregnancy when taken by women weighing 11st 11lb or more, according to HRA Pharma, which makes a pill containing the same key ingredient in France.
The average British woman weighs 11 stone, meaning that millions of those who seek emergency contraception could unwittingly, having taken the pill, still be at risk of pregnancy because of their weight.
The French firm is altering the packaging of its morning-after pill to warn of the new advised weight limit for drugs containing levonorgestrel.
A spokesman for the Medicines and Healthcare Products Regulatory Agency (MHRA) said that British officials would now look into whether the makers of Levonelle should do the same.
"The MHRA will review whether prescribing information for products approved in the UK containing levonorgestrel should reflect weight-related differences in efficacy rates, in light of the decision taken in France," said the spokesman.
Professor Anna Glasier, an expert in reproductive medicine at Edinburgh University, said that British women weighing more than 121/2 stone should seek alternative forms of contraception.
EllaOne, the other major morning-after pill brand available in Britain, contains a different active ingredient. A spokesman for Bayer, which distributes Levonelle, said: "Bayer Healthcare distributes Levonelle in the UK on behalf of the marketing authorisation holder, Medimpex. Any changes to the licence, if they are required, would be the responsibility of the marketing authorisation holder." Medimpex did not respond to a request for comment.
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