Chemistry series - Bromine
The tea is still scalding hot, so John leaves it on the counter to finish it after his shower.
It takes him six minutes longer than usual. Then he dresses for four minutes longer than usual. The deviation from John's routine disturbs Sherlock enough to lift his eyes from his experiments.
It's this woman John's been dating for two months. Mary something. Sherlock's annoyed to find he actually remembers her name. This woman is dangerous, and not the right kind of danger.
Sherlock focuses back on the brown liquid in his pipette. Halogen. Greek for 'salt producer'. Sherlock's mood is acidic, but his smile is sweet as a sugar when he drops the tiniest amount of light brown solution into the cooling tea. Dark brown Lapsang souchong. Tastes and smells horrible already, John won't notice anything amiss.
When John emerges again, his shave is flawless and he's wearing his best shirt. Clearly looking forward to getting bedded tonight. Just when Sherlock needs him for a case.
"Do something about that smell, Sherlock."
The recruits in military training in Eastern Europe during the Cold War always complained about the taste of their tea. They didn't know that the Army was keeping their libido fairly low for obvious reasons. No need to mess things up with transport.
A really useful substance, this bromine.