Metronidazole (and the related drug tinidazole) can interfere with how your body breaks down alcohol, leading to a build-up of a substance that causes a disulfiram-like reaction. Symptoms can include facial flushing, throbbing headache, nausea, vomiting, a fast heartbeat, and a feeling of being very unwell.
Because of this, you should avoid all alcohol while taking metronidazole and, as a precaution, for at least 48 hours after finishing the course (some advice suggests 72 hours for tinidazole). This also includes hidden sources of alcohol such as some mouthwashes, cough syrups, and tonics.
If you accidentally drink and feel severely unwell, contact a doctor or pharmacist. Seek urgent care for chest pain, severe vomiting, trouble breathing, or fainting.