Women on hormonal contraceptives such as the pill are being warned that they could fall pregnant if they take the herbal remedy St John's wort.
In the last six months the Medicines and Healthcare Products Regulatory Agency (MHRA) has received two reports of suspected interaction leading to unplanned pregnancies. The risk applies to the combined and progesterone-only pill and contraceptive implants like Implanon. It is unclear about the coil. The MHRA says there is no data to judge.
Since 2000, the MHRA has logged 19 suspected interactions with hormonal contraceptives - 15 resulting in an unplanned pregnancy and four in irregular menstrual bleeds.
St John's wort (Hypericum perforatum) is a herbal medicine traditionally used to relieve slightly low mood and mild anxiety. It is made from a plant with a bare trunk and yellow flowers, which grows mainly in Europe, Asia and North America.