The word could be toxic, or in its effect deleterious or harmful.
I had a bad headache; I took medicine and it antidote the pain.
The antidote will partially counteract the effect of the poison. The mayor tried to counteract the bad publicity that the city was receiving.
Pharmaceuticals is a Greek word (φαρμακευτικά). It derives from pharmaco (φάρμακο) meaning medicine and original meaning poison.
Offset.
Each sufferer has their own method to counteract their motion sickness.
Poison
Well there is counteract and react
The word "poison" has two syllables.
Because snake venom is deadly, someone had to create an antidote for it. There is no antidote for heavy metal poisoning, but there is a treatment. Your cheerful call was the antidote to a dreary afternoon.
Because it reverses the effects of something (anti means opposite)
Sportsmanship is a word substitute for spirit of game
There is no one-word substitute in English for "son-in-law."