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. ^ Low levels of menthofuran may not produce any untoward effects. At a certain point, however, depending on the individual, menthofuran poisons the nervous system, causing symptoms such as dizziness, vertigo, hallucinations, seizures, and possibly unconsciousness. Liver damage possibly leading to liver failure occurs subsequently.
Because of these safety risks, and the fact that pennyroyal has not been proven effective for a single medical use, we recommend that you entirely avoid this herb. We do not even recommend using it topically as an insect repellant, because it is possible that enough pulegone could be absorbed to cause harm.