This isn't so much a language question as a question of attitude towards addiction IMO. I would normally think of alcoholics and drug users falling off the wagon, where there would be a direct and obvious effect of their consumption, and would normally involve more than one sip.
However, would you feel the same way if an ex-smoker had a puff of a cigarette, or would they have to smoke a whole cigarette or a packet to fall off the wagon? It can be argued that any consumption involves a degree of falling of the wagon, but there's slipping a bit and falling right off it. If you get back on the wagon, then you can argue you certainly didn't fall right off. And good luck with quitting.