John is kidnapped by insert villain here and is injected with drugs, preferably cocaine. They don't beat him or torture him, just drug him for long enough that he becomes addicted. Eventually he is found by an extremely worried Sherlock and Lestrade. Sherlock deduces that he was given cocaine on a regular basis and is going to start withdrawal soon. John is taken back to Baker Street by Sherlock who takes care of him throughout the entire process. Afterwards, John starts to understand Sherlock's previous addiction and the random drug cravings.