Allergies to the medications, routinely used by dentists, are common. If an adverse reaction occurs, stop the use of medication and seek advice from your dentist. Reactions could be as minor as nausea and upset of the digestive track, or as serious as anaphylactic shock. Rash or hives are also a common allergic reaction. It is important to inform your dentist of all the allergies to medications and be vigilant, because mistakes can be made. If you have trouble breathing, you should seek out medical attention immediately.