98% of the time, the doc say dont drink on anti-biotics as the alcohol prevents the anti-biotics from working.. the other 2% (ish) is due to a reaction the alcohol has with the chems...
Yes, if you're on penicillin or similar, alcohol will make it much less effective. Probably best to play safe. Everyone will understand if you say "I can't drink, I'm on antibiotics."
I think the safest bet is to not drink. I remeber drinking one my last day of a course of amoxicillin, I got myself into a right state definately not worth it