In general, it is said that you should not mix antibiotics and alcoholic beverages and it seems prudent to do so, but the reasons behind it seems to be really fuzzy.
Some people say that the worse that can happen is that the antibiotics will not have effect. But other say that it will make you sicker. Some other say that both of these are true but it depends on the antibiotic. Some other say: "Just one won't hurt".
If someone could make this clear for us, it would be great.
QI provides the answer: with a few exceptions (where it will make you vomit) it is perfectly safe to drink alcohol whilst taking antibiotics.
Apart from some special circumstances, it is on the whole OK to drink while on antibiotics. The reason why people think that you should not is that when antibiotics was first being used it was to cure syphilis. The patients would still be infectious for the first week, so were told not to drink because they were more likely to have sex while drunk. This has since passed down and has become a "tradition" in a way. The main reason that people are told not to drink when on antibiotics is because doctors prefer people not to drink because it is better for them. However, the are some antibiotics like Flagyl which will make you vomit if you take them with alcohol.
MD-Tech provided a rather good answer. I suppose in theory you could also add to it the risk of infection if you are drinking an unpasteurized beer.
Normally you would have gut bacteria which would prevent the yeasts from colonizing your gut, but if you have lost those bacteria due to antibiotics then in theory you could get an infection. Of course, if you're still on antibiotics, odds are they will kill the yeast.
