The name pennyroyal refers to two related plants: Mentha pulegium (European pennyroyal) and Hedeoma pulegioides (American pennyroyal). Pennroyal is a member of the mint family. Applied topically, pennyroyal has been used since the time of ancient Greece to repel fleas and other insects. Pennyroyal has been taken internally in Europe and North America for a variety of conditions, including colds and flus, coughs, kidney problems, headache, and upset stomach, as well as to induce abortion. Unfortunately, traditional herbalists do not appear to have noticed an essential fact about pennyroyal: it is toxic to the liver. In modern times, people have died as a consequence of using this herb according to traditional indications.
The essential oil of pennyroyal contains a substance called pulegone. In the body, pulegone is converted to the toxic chemical menthofuran. 1