Depends. Depends on the antibiotic. Some antibiotics such as Metronidazole (flagyl) or trimethoprim-sulfamethoxazole (bactrim) can give a more severe reaction when mixed with alcohol leading to flushing, dizziness, fast heart beat, nausea and vomiting. Other antibiotics, such as doxycycline, are metabolized by the liver and may not be as effective when mixed with alcohol. If you have any specific questions about the antibiotic you are on, give your doctor a call.