Does poison have an expiry date too?

If it does, it would be printed on the bottle/package somewhere. Look for a “Sell by…” or “Good through…” or “Expiration…” date.

Who or what are you looking to poison, by the way?

Any chemical mixture – compound or single chemical – has a definite span of time over which the chemical(s) become less potent.

The trick is knowing the rate of loss of potency (in other words, it takes 6 months for Belladonna to lose half its strength) and the normal strength of the poison. And that’s going to be different for a compounds, poisonous or not.

It’s a similar principle – although not the same – as radioactive half life.

Chances are, it will just be poisonous in a different way.