When metronidazole is taken with alcohol, it can cause a disulfiram-like reaction, characterized by flushing, nausea, vomiting, headache, and tachycardia. This happens because metronidazole inhibits the enzyme aldehyde dehydrogenase, leading to the accumulation of acetaldehyde, a toxic metabolite of alcohol. The buildup of acetaldehyde triggers these unpleasant symptoms, similar to those experienced with disulfiram, a drug used to deter alcohol consumption. This reaction is important to recognize and avoid by advising patients to abstain from alcohol during and for at least 48 hours after metronidazole therapy.

Reference: Katzung BG, Basic and Clinical Pharmacology, 15th Edition.