Easily confused with :
Flower () [plants with similar names belonging to differing families]
Not to be semantically confused with : the Hydrocotyle Pennyworts such as Floating Pennywort (Hydrocotyle ranunculoides), nor with Penny-cresses such as Field Penny-Cress (Thlaspi arvense) or Wall Pennywort (aka Navelwort Umbilicus rupestris) [plants with similar names belonging to differing families].
Some similarities to : Corn Mint (Mentha arvensisn) but the sepal tube is markedly ribbed on Pennyroyal with hairs within the throat and the three upper sepal teeth being shorter and wider than the lower two.
The aroma is the strongest in the Mentha family, often to be judged repugnant by many, including insects - it can be used as a flea repellent. The taste too is often repellent. It has in the past been used as an abortifacient, and two women have been known to be killed by taking it. The oil is said to be toxic in 'large' amounts (whatever 'large' may mean...)! Contact of the skin with the oil can also cause dermatitis.
An accidental exposure of the oil to a dogs skin at 2g/kg weight led to the dog becoming listless and vomiting in the first 2 hours, and within 30 hours exhibiting diarrhoea, haemoptysis (coughing up blood) and epistaxis (nose-bleed), developing seizures and dying. The toxin is thought to be Pulegone which is metabolised in the body to Menthofuran which in turn is transformed by CYP enzymes to a compound which is hepatotoxic (toxic to the liver).
The main constituents of the essential oil of Pennyroyal are Menthone (at 31%). Others include Pulegone (14%),
NeoMenthol (14%) (although the percentages vary wildly from the various sources to be found on the internet), and Caryophyllene Oxide, according to one source. Other volatile compounds are
Pulegone Acetate, Caryophyllene, γ-Terpinene and another 35 or so unspecified but more minor constituents.
NeoMenthol is the simplest of these three, an alcohol. It has a mentholic type odour and flavour and is found in many foods, such as Broccoli, Sweet Orange, Yellow Bell Pepper, etc. Speaking personally, your Author has never detected any menthol-type aroma emanating from brocolli, either when cooking or upon eating. It is used as both a flavouring and as a fragrance.
Piperitone is a ketone and is found in nature in both its' possible stereoisometric forms: D- and L- forms. D-Piperitone has a peppermint aroma and is the form found in some plants of the Mentha genus. L-Piperitone, on the other hand, is found in
Sitka Spruce and the Eucalyptus dives tree but probably smells dissimilar.
Piperitenone is found in the citrus fruits such as Grapefruit, Lemon, Orange and also in Spearmint and Peppermint. It is used as a flavouring agent. It is a
p-Menthane monoterpenoid (as are the others).