Propylene Glycol is used as a base carrier for the nicotine and flavoring. Propylene Glycol has been deemed generally safe to be used in medical products as well as food products for general consumption. Some people have reported having an allergic reaction to PG. These people also find that they have issues with certain brands of toothpaste, lotions, soaps, and medicines containing PG. If you have issues with PG, it is suggested you look into using a product with a Vegetable Glycerin base.