Like all mint family plants, pennyroyal owes much of its medicinal activity to the presence of a volatile oil. The primary component of this oil is known as pulegone. Pulegone is converted to menthofuran by the body. If large enough amounts of pulegone are consumed, the amount of menthofuran produced can seriously damage the liver and nervous system.³ Smaller amounts of the volatile oil contained in the whole plant appear to have mild, smooth, muscle-relaxing effects that might help explain the historical use of pennyroyal for indigestion, stomach cramps, and cough.⁴ No modern clinical trials have been completed to support these indications, and other herbs with soothing effects on the gastrointestinal tract, such as chamomile and peppermint, have a much greater history of safety than pennyroyal.