[ There is no competition. Sherlock knows that Watson would be best boy. He can settle with being Mrs. Hudson's second best boy when first is his Watson.

Sizhui entertaining his poison talk is only encouraging Sherlock to continue. His eyes light up at someone showing interest. It distracts him from preparing more coffee for the moment. Drinks for the banquet have been forgotten. Now is time for poison talk while in the kitchen.

(Woe for anyone else in the kitchen listening to them, and hopefully, Sizhui's food preparation isn't suffering.) ]

There is a problem with using more than the desired dose as well. Let's say the target is a fast and voracious eater. Too much of the additive can cause a covert break-in to become a murder scene.

[ Which then creates another set of problems. ]