Antidotal

a medicine or other remedy for counteracting the effects of poison, disease, etc.
something that prevents or counteracts injurious or unwanted effects:
Good jobs are the best antidote to teenage crime.
to counteract with an antidote:
Medication was given to antidote the poison the child had swallowed.
Historical Examples

The knowledg of these antidotal Herbs they have learned from the Mounggoutia a kind of Ferret.
An Historical Relation Of The Island Ceylon In The East Indies Robert Knox

Its juices were said to be antidotal to snake poisoning, and would also cure the opium habit.
Alcohol: A Dangerous and Unnecessary Medicine, How and Why Martha M. Allen

The woods might have been filled with antidotal remedies, and I have died in their midst.
The Quadroon Mayne Reid

Your letters would be antidotal, and thus, by a sort of mental allopathy, beneficial.
The Jessica Letters: An Editor’s Romance Paul Elmer More

It is curious that among the non-venomous animals the rabbit’s bile is the most powerful in antidotal properties.
Appletons’ Popular Science Monthly, January 1899 Various

noun
(med) a drug or agent that counteracts or neutralizes the effects of a poison
anything that counteracts or relieves a harmful or unwanted condition; remedy
adj.

1640s, from antidote + -al (1).
n.

“remedy counteracting poison,” 1510s (earlier in English as a Latin word), from Middle French antidot and directly from Latin antidotum “a remedy against poison,” from Greek antidoton “given as a remedy,” literally “given against,” verbal adjective of antididonai “give in return,” from anti- “against” + didonai “to give” (see date (n.1)). Cf. Middle English antidotarie “treatise on drugs or medicines” (c.1400).

antidote an·ti·dote (ān’tĭ-dōt’)
n.
An agent used to neutralize or counteract the effects of a poison.
an’ti·dot’al (ān’tĭ-dōt’l) adj.
an’ti·dot’al·ly adv.
antidote
(ān’tĭ-dōt’)
A substance that counteracts the effects of a poison.