Endodontic materials can cause tissue damage which will lead to inflammatory responses in soft tissues. These responses may interfere with the healing of apical periodontitis or serve as a locus for colonization by microorganisms to create a lesion where none existed. Any chemical compound will have a concentration gradient in the tissues with bacteriocidal and then bacteriostatic activity, but the cytotoxic effects will always be wider-ranging. Experiments with cell culture techniques and toxicity tests in animals have aided in our selection among chemicals and materials for endodontic use. Moreover, the allergenic and genotoxic properties of medicaments must form part of our selection criteria.