Tired of being subjected to Sherlock's experiments without his own consent, John decides to teach Sherlock a good lesson he won't forget any time soon. Sherlock is too distracted by a case to notice that his drink is spiked with a strong diuretic and laxative. He ends up pissing and soiling himself at a very inappropriate moment and place. Maybe nobody notices and Sherlock leaves the place before anyone realises, but John, of course, knows what has happened. He helps Sherlock to get home and probably also helps him to take care of the disaster.

The lesson is learned though, and Sherlock never uses John as his test subject again without John's explicit consent.