Why can't you drink alcohol while taking antibiotics? what is the main reason?

Possible reaction: Some common antibiotics, like metronidazol (flagyl), trimethoprim-sulfamethoxazol (bactrim), and tinidazol (tindamax) can cause severe flushing, headache, nausea and vomiting, and elevated heart rate when combined with even a little alcohol. In general, the side effects of other antibiotics are similar to those of drinking, and might therefore be worsened by consuming alcohol.

Antabuse (disulfiram) reaction: There's only one antibiotic that causes a potentially serious reaction when mixed with alcohol and that is Flagyl (metronidazole), which only happens to some people. Its most common use in the out-patient arena is for bacterial vaginosis so women watch out! essentially, you can get very sick when you mix the two; although i've never seen it, fatalities have been reported, so please be careful.