Both the oil and the extract can have beneficial properties, but they can also have a sensitizing effect on skin. Peppermint oil is a more potent sensitizer than peppermint water. Peppermint is a common cause of sensitivity on the face, particularly around the mouth, as it’s frequently used as flavoring in toothpastes. The oil contains 44% menthol, an ingredient believed to be largely responsible for peppermint oil’s benefits as well as its risks.
References for this information:
Asian Pacific Journal of Tropical Biomedicine, August 2015, pages 601-611
Contact Dermatitis, April 2008, pages 143-150 and January 2008, pages 9-14
Seminars in Cutaneous Medicine and Surgery, Volume 32, 2013, page 143
Basic and Clinical Pharmacology and Toxicology, June 2006, pages 575-581