What are two non lethal substances that will combine to create a strong poison when ingested close together?
Hopefully my character would be able to drink one of the substances mixed in to a wine or whiskey that is shared out with a few people, but does not kill anyone (yet!). Soon afterwards the character goes upstairs and takes some form of pill, which should have the other half of the poison in it. The pill could have been switched out weeks in advance, without detection. This combination kills them - but as multiple people drank the wine, and they have been taking the pill for days without being poisoned, it is difficult to piece together how they have been killed.
Preferably the two ingredients would be naturally occuring as it is a low-tech world, and without taste to help avoid detection, but I know that's very unlikely so no worries if it has some taste/smell! I'm sure I can find a way to play it off!
I have thought of sulphur tablets with Isopropyl aminoethylmethyl phosphonite in the wine, which creates a VX (a strong nerve agent), however I'm having trouble thinking of a way of the character being able to get hold of Isopropyl aminoethylmethyl phosphonite with only fantasy technology... Am also unsure of the quantaties needed etc.
Another option is an overdose of a certain chemical, with one half being in the wine and the other in the pills, but again I'm unsure of quantity and what would work best.
Edit: sorry forgot to say, the context is a murder mystery story in a fantasy world.
Thank you all so much!