What a tangled web we weave....This is a case where honesty may have been the best answer but it is hard to know what to do some times.
You can compound the lie in a week and tell him the antibiotics were not working and the doctor ordered a new regimen but that does not get you out of the lunch. It may get postponed, again!
So why you can use the new drug excuse I would also suggest you encourage the lunch to happen ASAP or just flat out tell him you have stopped drinking and would prefer a non-alcoholic lunch.
Better yet, you could explain that since you are not drinking that it will save him money and with that money he could just get some pizza for the whole office to share instead. Win WIN!