So my SO (significant other) and I are having a discussion about the meaning of "fall off the wagon". Ten weeks ago I quit drinking Coke - something I was heavily addicted to, averaging half a gallon p/day. Today I had a sip of a Coke that my SO had with him and he says that I fell off the wagon. I say one small sip does not count as falling off the wagon, that I would have to start drinking it again for me to actually fall off the wagon. He says that it doesn't matter how much or how little I ingest - that to fall off the wagon is to use something that one has vowed to never use again, even if its only just once.
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.