Resin-based materials are used extensively in restorative and preventive dentistry. Because they do not completely polymerize, unreacted monomers can leach out of resin matrices with potential adverse effects for patients. The amount of leachable components is typically greater for undercured resins. On rare occasions, individual patients have experienced skin or mucosal reactions to resin- based materials, including anaphylactoid or lichenoid reactions and other allergic responses. However, in most reported adverse reactions, the symptoms subsided on removal of the restorations, sealants or appliances containing resin-based materials.