PENNYROYAL Overview Information. Pennyroyal is a plant. The oil and leaves are used to make medicine. Throughout history, both American pennyroyal and European pennyroyal have been used interchangeably as a source of oil.Despite serious safety concerns, pennyroyal is used for colds, pneumonia, and other breathing problems.It is also used for stomach pains, gas, intestinal disorders, and liver and gallbladder problems.Women use it to start or regulate their menstrual periods, or to cause an abortion.espite serious safety concerns, pennyroyal is used for colds, pneumonia, and other breathing problems. It is also used for stomach pains, gas, intestinal disorders, and liver and gallbladder problems.