I am of a certain age and don't take medications, so I understand the character. Let's see,
poisoned biscuits that someone gives to him? poison tea?
his shaving cream is poisoned? kills on skin contact?
mistakenly forages wild poison mushrooms?
anaphylaxis to cats? dogs? wasps?
steps on nail and gets lockjaw? or an infection he can't treat because the hospital is in lockdown?

Good luck.
NJ