Pennyroyal is a powerful emmenagogue causing the uterine muscles to contract. It is typically combined with other herbs to help stimulate delayed menses. It is therefore not recommended for pregnant women. The herb also contains pulegone, a highly toxic compound when used in high or concentrated doses. It can cause severe liver and kidney damage. Never use the essential oil of pennyroyal internally as it is a highly concentrated form of the herb and is therefore extremely toxic, possibly fatal. The fresh or dried leaves used in tea are considered safe in proper doses. American and European pennyroyal have been used interchangeably as a source for the oil.