Substance that counteracts one poison but accelerates/exacerbates the effect of another?

Is there an example of substances interacting with each other in the following way?

• A is a lethal substance, killing within minutes to hours when orally ingested;
• anti-A is an antidote for A (or at least significantly counters its effect), works even if administered (orally) a few minutes after being exposed to A;
• B is a lethal substance, killing within minutes to hours when orally ingested. Initially does not present with severe symptoms. However, administering anti-A after being exposed to B accelerates/exacerbates the effect of the latter, causing the death to occur virtually instantly.

Bonus points if A and anti-A are naturally occurring substances. As this is going to be a part of a D&D session, answers with A, anti-A and B belonging in either real life or the D&D universe are also welcome (if some of the required aspects are not canonically established, a brief medico-biological justification for why they might plausibly interact in this way is expected). If I can think of any other pertinent points, I will elaborate.

Edit: Thank you all for your suggestions! I settled for a minor development of Willk's answer: namely, replacing sarin with onchidal (https://en.wikipedia.org/wiki/Onchidal). The underlying biological mechanism should remain largely the same, as far as I can tell.

posted over 4 years ago

A Y Mullion‭

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