Alcohol may cause antibiotics to be less effective. But some say this is a myth. I believe it depends on which antibiotics you are on.
http://www.gpnotebook.com/cache/-946208714.htm
There is a generally held belief that alcohol should not be consumed while on a course of oral antibiotics. This advice is correct if a patient is taking metronidazole, cephamandole (may cause a disulfiram-like reaction), and possibly oral ketoconazole (may cause facial flushing). However there is no contraindication to the consumption of alcohol with other antibiotics.
"For almost all antibiotics, drinking alcohol is no problem. Only metronidazole (Flagyl) produces a number of unpleasant side-effects when taken in combination with alcohol. This product is prescribed for certain sexually-transmitted diseases and vaginal infections. You might suffer from loss of appetite, nausea, headaches and, sometimes, a red face. This possibly also applies for ketoconazole (Nizoral). Certain other medicines also cause problems in combination with alcohol."
http://www.bierengezondheid.be/index_eng.jsp?Page=Doc437&Doc=forum
http://www.drweil.com/u/QA/QA326593/