Pennyroyal (Mentha pulegium) essential oil has gotten a bad reputation. Decades ago it was deemed unsafe, and many Aromatherapists have not changed their opinion of it. It is an often avoided oil, but it can be used if safety guidelines are followed.
The warning against Pennyroyal probably stems from “a case in the USA in which a woman drank a large dose of pennyroyal in order to induce an abortion that proved fatal to her. … There are two other cases in which women consumed the same large doses of pennyroyal (100 to 200 times the recommended topical application) in which both the mothers and the babies survived.” Keep in mind that in these instances the essential oil was taken orally, a method of using essential oils that carries its own dangers. Also, “the large doses needed to cause an abortion can kill the mother or cause her irreversible kidney and liver damage.” All in all, it’s not a sane or safe way to use any essential oil.
The best way to get the benefits of Pennyroyal is as a flea and insect deterrent. It is safe for dogs and horses, but not cats, and can be added to shampoos or sprays. Another use is via inhalation for asthma and lung congestion. The main caveat is to avoid in pregnancy as it is a uterine stimulant, which is probably why it was chosen as an abortifacient. You should also use it at no more than 0.2% in an external application.
Now that I have peeled away the warnings (and their detractions) that surround this oil, I should reveal that I don’t use it. The first time I smelled it I began to gag violently. I realized that I would trust what my body was telling me and I have stayed away from the oil. So, whether you use Pennyroyal or not, it should be based on your knowledge of (or reaction to) the oil and be your choice.