My husband has dementia. He believes he still smokes. He has not had a cigarette in 24 years. He gets agitated and accuses me of hiding them.

I’m really sorry you’re facing this, confirm his feelings and avoid arguing, and gently redirect him to a calming activity or substitute instead of focusing on the cigarette.

That’s really common with dementia. Don’t argue or try to prove he’s wrong, it usually makes agitation worse. Reassure him and redirect his attention instead. It’s the disease, not him, and you’re not doing anything wrong.