Complications of Mixing Flagyl (Metronidazole) With Alcohol

Flagyl is the brand name of metronidazole, an antibiotic used to treat a variety of infections, from stomach ulcers to meningitis. (Metronidazole is also found in some other brand name antibiotics, including ; check your antibiotic's label.) Most of Flagyl's side effects are run-of-the-mill, such as nausea and vomiting. However, Flagyl is reported to interact dangerously with another common drug: alcohol.

Drinking even a small amount of alcohol (ethanol) while taking Flagyl can make a person very sick. Flagyl and alcohol together cause severe nausea and vomiting, flushing, fast heartbeat (tachycardia), and shortness of breath. The reaction has been described as being similar to the effects of Antabuse, a drug that treats alcoholism by causing patients to become very sick when they drink.

Obviously, beverages containing alcohol should not be consumed during treatment with Flagyl, but small amounts of alcohol can be found in hidden sources as well. Some kinds of mouthwash and cold medicine contain alcohol. Small amounts may also be served at religious services. Patients should avoid all of these alcohol sources while taking Flagyl and for 48 hours following the end of treatment.

Because the Flagyl-alcohol reaction is said to resemble the Antabuse-alcohol reaction, researchers originally assumed that they work the same way. Ordinarily, the liver breaks down ethanol in two steps: first into acetaldehyde, then into acetic acid. Antabuse inhibits the second step, causing levels of acetaldehyde in the blood to rise. The increased blood acetaldehyde causes the acute symptoms of vomiting, flushing, etc.

More recent research has shown that Flagyl does not inhibit the breakdown of acetaldehyde, and that blood acetaldehyde does not increase when Flagyl and alcohol are combined. Therefore, some other mechanism must be at work. One set of researchers (Karamanakos et al. 2007) suggested it may be related to increased serotonin because they were able to show that Flagyl increases brain serotonin in rats. Another set of researchers (Visapää et al. 2002) noted that there are only 10 human case reports of a bad Flagyl-alcohol reaction and suggested that the problem may not be as common as previously thought. They did, however, note that it is possible that this "reaction can occur in some subgroups," so it is still wise to avoid mixing Flagyl and alcohol.

• P.N. Karamanakos, P. Pappas, V.A. Boumba, C. Thomas, M. Malamas, T. Voigiouklakis, M. Marselos. "Pharmaceutical agents known to produce disulfiram-like reaction: effects on hepatic ethanol metabolism and brain monoamines." International Journal of Toxicology 2007 Sep-Oct; 26(5):423-432.
• James M. Steckelberg, M.D. "Antibiotics and alcohol: Should I avoid mixing them?" 2008, MayoClinic.com.
• J.P. Visapää, J.S. Tillonen, P.S. Kaihovaara, M.P. Salaspuro. "Lack of disulfiram-like reaction with metronidazole and ethanol." Annals of Pharmacotherapy 2002 Jun; 36(6):971-974.