It is most often observed as a sign of tetanus. It can also be caused by poisoning with strychnine or Wilson's disease. In 2009, scientists at the University of Eastern Piedmont in Italy wrote that they had identified hemlock water-dropwort (Oenanthe crocata) as the plant historically responsible for producing the sardonic grin. This plant is the most likely candidate for the "sardonic herb", which was a neurotoxic plant used for the ritual killing of elderly people in pre-Roman Nuragic Sardinia.
Risus sardonicus
Arnica montana
Lathyrus sativus
Brachyspira pilosicoli
Cynanchum viminale
Baylisascaris shroederi
Hirudo orientalis
Chrysomya putoria