It is revealed that Johnson was an alcoholic for 15 years, and like many people he believes that recovering alcoholics should completely abstain from drink. However in "Mark Makes a Friend" (his first appearance in series 1) he is shown drinking a glass of wine while having dinner with Mark in one scene, and a pint of lager when he meets him in a bar in another scene. If he was an alcoholic at that point, he probably wouldn't be able to easily have one casual drink, if he had given up drinking, he wouldn't be drinking at all.