Not only does it affect it but depending on the one given you can be very sick from it.

Since you didnt say what it was exactly or why -I chose a few...

Metronidazole (Flagyl) Alcohol consumed with this antibiotic can cause severe reactions of nausea, vomiting, diarrhea,GI bleeding, liver damage, seizures. This antibiotic has a part of the molecule that acts like Antabuse and prevents the alcohol from completely being metabolized and a build up of the toxins can make you violently ill or could possibly be fatal.

2. Sulfonamides (Bactrim,Septra) can cause a similar reaction with alcohol as metronidazole in some people. Nasea and Vomiting.

3. Zithromax, Biaxin, Erythromycins can be fairly hard on the liver to eliminate. Livers damaged by chronic alcohol may intensify this greatly. Competition for elimination may may drinking excessive alcohol fatal. Telithromycin (Ketek) in addition can potentiate the dowsiness caused by alcohol.

4. Tetracyclines/Doxycyclines can cause GI upset. Alcohol also causes excessive stomach acid to be released increasing the GI distress to the point of vomiting.

5. Nitrofurantoins (Macrobid, Macrodantin) cause GI upset worsened by alcohol. Nausea and Vomiting.

6. Augmentin can cacuse GI upset worsened by alcohol. Nausea and Vomiting

7. Quinolone antibiotics (Cipro, Levaquin) GI upset made worse. If the elimination of the antibiotic reduced significantly causing excessive blood levels, changes in the QT interval of the heart beat can cause it to fail. This can be true of the Erythromycins too. Nausea and Vomiting.

Most antibiotics are dosed at 7 to 8 times what is necessary to lethally kill the bacteria that are supceptible to it. Decreased absorbsion with most I am aware of only marginally change their effectiveness with alcohol. The main concern is intensifying side effects or harm caused to the patient rather than decreased effect of the drug.