It can interact with some medicines. This can have harmful interactions and make the medication not work properly. You should also know that peppermint oil can inhibit the metabolism of certain drugs. If you are taking any medications, you should not take Peppermint oil. It can even cause side effects. To avoid any unpleasant side effects, check with your pharmacist. If you have any doubts, consult your doctor. You can use a product that contains a preservative that is free of preservatives.