“Note that ‘American Pennyroyal’ or ‘false pennyroyal’ is Hedeoma pulegioides. Many aromatherapists will not use this oil because it is an oral toxin and it is considered an abortive. Use care and never in large amounts. It can be used in a massage blend for the pelvis as it can induce menstruation and relieve that congestion. The oil is an insect repellant and can be used in the bedding of dogs and cats. Dilute in alcohol or vinegar and use as a spray. One or two drops to a steaming pot of water for the person suffering from congestion of the lungs, bronchial infections and the painful coughing that comes with that deep chest cold

A 2010 study found that Red Thyme, Lemon Eucalyptus, Citronella, Pennyroyal, Caraway Seed and Cloverleaf essential oils were effective against red spider mites that had become resistant to chemical pesticides.

http://www.ncbi.nlm.nih.gov/pubmed/20857739”