Dental materials used in treatments must be functional as well as biocompatible, that is, harmless to all oral tissues and they mustn't provoke (cause) adverse effects. These effects can be subdivided into: systemic toxicity, local reactions, allergic reactions and other reactions. Before a material is approved for market launch, it must undergo specific types of testing: primary testing that evaluates cell response, secondary testing that evaluates tissue response, then experimental testing on animals and, finally, clinical testing on people. This paper describes possible toxicity of dental amalgams and their effect on oral cavity and other organs and systems. It shows the constitution of composite and adhesive systems and elements that can potentially be toxic. It describes the constitution and classification of glassionomer cements and the impact that their constitution has on tissues. The paper also presents various irrigants for root canal desinfection, intracanal medicaments and root canal filling materials. Finally, it describes the appliance of fluorine in the prevention of dental caries, as well as harmful effects that fluorine can have on teeth.