The toxicity of endodontic materials is reviewed. The patient may be exposed by constituents leaching from the unset and set sealer and by accidental overfilling of the root canal. For most patients the exposure is believed to be below the threshold for a toxic effect. Most sealers exhibited a cytotoxic response, especially unset and the responses diminished with time after setting. Unset and newly set endodontic material may also have a toxic potential in vivo that may result in a localized inflammation postponing the healing of an apical periodontitis. Root canal sealers containing paraformaldehyde is of biological concern.

Reference 
Toxicity of endodontic filling materials.
Dahl JE.
Endodontic Topics 2005;12(1):39–43.