on the wagon
adjective
°Abstaining from drinking any alcoholic drink, usually in the sense of having given it up (as opposed to never having partaken).
"1917: “Thank you, but; – er – I’m on the wagon, you know,” declined the youth. — Edgar Rice Burroughs, The Oakdale Affair"
°By extension, maintaining a program of self-improvement or abstinence from some other undesirable habit.
synonyms: teetotal