blood.

Being ever the doctor, John does their blood work himself, locating a cooperative vein in Sherlock's left arm that has obviously yielded results before, judging by the level of scarring. He glances up at the other man while drawing the first tube, finding him looking endlessly uninspired.

Not a flattering look.

"I can tell I wasn't here first," John comments, referring both to old needle marks and old lovers.

Sherlock responds with the eloquent quirk of an eyebrow, more interested in whether John the Second has passed his potential gonorrhea on to the both of them, neither or just John.