For the medieval period, the word is attested once in one of the manuscripts of a Continental French version of the Roman d’Alexandre (see TL tossin 10,469). TLF has
toxine and OED
toxin n., both dated from the 19th c. The implied etymon *toxinus is not attested in Latin.
s.
1poison, toxic substance:
(
MS: s.xiv1
) La treszime herbe [...] si ele soit mys desouz sardoigne ové poy de toccyn (Latin: um parvo de toxico), fra le portaunt bien entendaunt
188
[toxin] (s.xiv1)