Not drinking on antibiotics is one of those common-sense health rules that's a fact just because, like waiting 30 minutes after eating to swim, or peeing on a jellyfish sting to make it better.
It's also a great excuse to use if you don't feel like going out one night; most people will accept it as legit.
But is it actually true? After all, people certainly do it, and while there are some anecdotal stories about feeling the alcohol kick in harder, it's not like people are going to the hospital in droves after mixing the two.
The advice dates back to the (horny) 1950s
When antibiotics were invented, they were literally super-drugs. Suddenly, previously devastating illnesses, from common hospital infections to syphilis, could be cured in a matter of days or weeks. Needless to say, this drastically changed prescription drugs and how patients interacted with them.