This section is from the book "Botanic Drugs Their Materia Medica, Pharmacology and Therapeutics", by Thomas S. Blair. Also available from Amazon: Botanic Drugs, Their Materia Medica, Pharmacology and Therapeutics.
Pennyroyal (American), Hedeoma pulegioides. Official in the Eighth U. S. P., as was also oil of pennyroyal. In Mexico the flowering herb is official. It is a gentle aromatic stimulant useful in flatulent colic and used in infusion. Large doses of the oil possess narcotic properties of no known therapeutic usefulness. A spirit of the oil is repellent to fleas and mosquitos, and is applied to the skin and face as the lesser of two evils - by those who consider it such. Pennyroyal has no specific influence upon the womb and ovaries, except as noted under "Helonias," q. v.