Women using the morning after pill may need a double dose to avoid conceiving if they are taking other drugs.
Medicines for epilepsy and common infections – as well as some herbal remedies – can interfere with the emergency contraceptive and make it ineffective.
The morning after pill (known as Plan B in the US) contains the progesterone levonorgestrel.
It stops the ovaries releasing eggs and irritates the lining of the uterus, making it incredibly difficult for a fertilized egg to implant.
But when paired with common anti-seizure medication such as phenytoin, or herbal remedies to ease stress, the hormone to kick-start this process is drowned out.