I found these tags which should IMO be synonyms of each other or merged together:
toxicity (19 questions, 1 follower)
Excerpt: Questions related to being poisonous; relating to or caused by poison(s).
toxins (6 questions, 0 follower)
toxicology (2 questions, 0 follower)
Simply looking at their stats, toxicity is most popular and should be the master tag.
On the other hand, Biology.SE has a
toxicology tag only, so making that one the master tag would make it easier for people coming from there.