I think the real question here is WHY does Sherlock keep poisonous things in the kitchen?
It’s canon. Sherlock Holmes has always kept dangerous stuff in the kitchen. Watson is all “clean the place up, Holmes.” but then Holmes distracts him with a flashback and we never find out if Holmes cleaned up.
Our chambers were always full of chemicals and of criminal relics which had a way of wandering into unlikely positions, and of turning up in the butter-dish or in even less desirable places. - The Musgrave Ritual