www.alphadictionary.com

There is a word which describes the medical treatment which ameliorates the side effects of a primary treatment.
e.g. chemotherapy drugs cause nausea so another drug is proscribed to treat the nausea.

Pertaining only to drugs I think.

The word is in my head somewhere but the passing years take the toll on my ability to recall.

Would mitigative or mitigatory be what you're looking for?

ameliorate, amelioration, ameliorant, ameliorative, amelioratory may useful

All the suggested words could possibly apply but are not specific to counteracting medicine's side effects.

The word I am seeking is medicine specific, at least in my mind that is.

Imagined word? Perhaps but I am convinced not.

I googled "effect of another drug", and found "antagonism" and "antagonist". Could that be what you are looking for?

No not either of those, Audiendus.
They are antonyms of agonist and agonistic? i.e. effect, (or not), of how they affect cell release of say, dopamine

Two more suggestions:

antidote
palliative

Counteractant, counteragent? If not, try trolling the xynonyms for antidote. That's where I found those two.

Thanks, good people.
Not the word I have in mind.
I am mistaken, perhaps.

It's becoming my sole ambition!

I think you are correct. There's a tickle in the back of my brain that sometimes brings up the right word. Meanwhile, I plan to ask the next pharmacist I see.

I think you are correct. There's a tickle in the back of my brain that sometimes brings up the right word. Meanwhile, I plan to ask the next pharmacist I see.

Great idea! Now why didn't I think of that

I think you are correct. There's a tickle in the back of my brain that sometimes brings up the right word. Meanwhile, I plan to ask the next pharmacist I see.

Great idea! Now why didn't I think of that

May you have better luck than I. 3 pharmacists and 3 nurses later, I have diddly.

Unaccustomed as we word mavens are to field work...

Next step a real medical man then. What some choose to name a 'doctor'.
Though I do wonder whether, even if this doctor knows the word I am seeking, it would be revealed.

You fear that if he tells you, he'd have to kill you? :-)