The name Pennyroyal is a corruption of the ancient herbalists' name 'Pulioll-royall' (pulegium regium) which meant is was good against fleas. It was also called 'Piliole-rial' in the Middle Ages. It was used medicinally as an antiseptic, antispasmodic, emmenagogue (to stimulate menstrual flow), carminative, stimulant and for asthma, bowel disorders, pneumonia and skin eruptions.
Pennyroyal was also applied to the skin to kill germs, repel insects and as a treatment for gout, mouth sores, venomous bites and as a flea-killing bath. Pennyroyal oil is still sometimes used as a dog and cat flea repellent and a fragrance for detergents, perfumes and soaps.
However, internal use can cause abortions, liver damage and it is a neurotoxin. Even a tea made from it has been known to kill infants and dogs have died after licking their fur to which it has been applied.
Consumption of pennyroyal can cause abdominal pain, dermatitis, increased blood pressure and pulse rate, lethargy, nausea and vomiting ands should be avoided.
With this in mind the only good use of pennyroyal is as a companion plant to repel insects and beetles from food crops such as the aubergine. Fresh flowers and branches can be used to repel insects and fleas around the home but the oil should be avoided. Pregnant women should avoid it altogether.