Pennyroyal is the name given to two species of mint. The European species, Mentha pulegium, is the usual source for its medicinal use, but the American pennyroyal (or false pennyroyal), Hedeoma pulegoides, produces the same chemical, pulegone, that accounts for both its fragrance and its toxicity.
We haven't grown pennyroyal. We avoid all type of mint because they are quite aggressive in the favourable conditions of a well-tended garden. The European species is recognized as invasive through most of coastal California and Oregon. If you do want to grow any type of mint, you should keep it in a container adequate to restrict the stolons, which can advance rapidly underground before you recognize that you have a problem.
Pennyroyal has a long history of use in traditional medicine and, as is true for many herbs, numerous therapeutic claims have been made for it. Modern claims and uses are in two main categories. It is promoted as both an insecticide and insect repellent as well as an emmenagogue (promoter of menstruation) and abortifacient.
Many fragrant plant extracts have been applied to the skin as mosquito repellents, perhaps the best-known being citronella, which is derived from lemon grass. There are many reports in the medical literature showing the efficacy of these substances, including pennyroyal, but their methodology is so variable that they are difficult to evaluate. Some of the more convincing studies involve measuring the time to ‘first bite’ when volunteers are exposed to a mosquito-infested environment. A more complex method placed subjects in a wind tunnel into which hungry female Aedes aegypti were released. Efficacy was judged not from whether the subjects were bitten but by the tendency of the mosquitoes to be attracted to them. Citronella candles, as well as a sonic wearable device, did not affect attraction. No comparable experiment has been done with pennyroyal, whether applied to the skin or as aromatherapy.
Until recently, applying a few drops of pennyroyal oil to your dog’s coat was promoted an effective repellent for fleas and ticks. It was not recommended for cats because they might ingest the oil while self-grooming. Though a case of fatal poisoning of a dog treated for fleas with pennyroyal oil was reported in 1992, the information was slow to reach the Google-sphere. Today most websites caution against using pennyroyal on pets.
The use of pennyroyal and pulegone to promote abortion goes back to classical times, and numerous instances of human poisoning, some of them leading to death, have been reported. Three cases, one fatal, were reported from Denver (JAMA 1980 Apr 4;243:1365) at a time when legal abortion was available. With the new restrictions on interruption of pregnancy we shall probably see many more. You can readily buy pennyroyal on Amazon as dried leaf, liquid extract, and essential oil. Some of the offerings provide cautions about its toxicity or warn against internal use. However, there are also capsules clearly intended to be taken by mouth. Customer reviews include comments such as “started my period … so it works wonderfully” and “I drank one cup and bam. My period came around 4 hours later.”
The serious and lethal consequences of pulegone ingestion are due mainly to its toxic effect on the liver. The massive necrosis (death) of liver cells bears some resemblance to that seen in acetaminophen (Tylenol) poisoning and, perhaps for that reason, acetylcysteine has been recommended as a treatment. The few case reports of human and animal poisoning where it has been used are difficult to interpret and, surprisingly, I found no articles on MedLine reporting its use on an experimental model of pulegone poisoning.