Drinking alcohol while taking metronidazole is not recommended because the combination of metronidazole and alcohol can cause a reaction (often referred to as a disulfiram-like reaction) in some people. Symptoms may include flushing, headaches, nausea, vomiting, and stomach cramps. There has been one reported death associated with this reaction. The product information and health professionals recommend not to drink alcohol during metronidazole treatment and for 3 days after finishing the course.

However, there is controversy around this reaction because while some studies have shown serious problems for some people taking metronidazole, others have shown the combination does not cause any problems. Large clinical trials in humans have never been conducted to confirm this interaction.

The reaction has been referred to as a disulfiram-like reaction – disulfiram is a medication given to people to discourage alcohol consumption. When a person consumes alcohol, the body breaks it down in two steps. First, it breaks alcohol down into a compound called acetaldehyde. Acetaldehyde is responsible for the unwanted effects of alcohol such as nausea, vomiting, and flushing, and is toxic. Next, the body reduces acetaldehyde to acetate using an enzyme called aldehyde dehydrogenase. Acetate is easily oxidized by our body into carbon dioxide (CO2), which we then breathe out. Disulfiram blocks the effects of this enzyme which leads to acetaldehyde accumulation causing symptoms such as skin redness, palpitations, nausea, vomiting, headache and in severe cases a dangerous rapid heart rate or a sudden drop in blood pressure. It was thought that metronidazole blocked the enzyme aldehyde dehydrogenase as well, although this now seems to be incorrect.

Several studies that have investigated the reaction of metronidazole with alcohol have found evidence of the existence of this interaction to be absent or weak. It does seems that the concern attached to this reaction is overstated. It is possible that the reaction could just be a side effect of metronidazole or potentially only occur in a small sub-group of susceptible people, because the reaction does not appear to occur in everybody.

There needs to be more research done investigating this potential interaction, but because doctors are unable to say which people are more at risk of this interaction, it is best to err on the side of caution and avoid alcohol while taking metronidazole until more is known.