Metronidazole: doses, uses, and side effects

Metronidazole (brand name Flagyl) is one of the go-to antibiotics for treating a wide variety of bacterial and parasitic infections. The drug comes in several forms, including oral tablets (immediate and extended-release), capsules, creams, and a vaginal gel. It also comes as an injectable medication that a healthcare provider can give intravenously (IV) for more serious infections.

Metronidazole works by stopping the growth of the microorganisms causing the infection. The drug enters the cells of the bacteria or parasites that cause the infection and interacts with the organisms’ DNA. As a result, the organisms can no longer multiply, which relieves the infection. 

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The Food and Drug Administration (FDA) approves metronidazole for the treatment of a broad range of infections, including infections of the stomach, liver, skin (acne), joints, brain and spinal cord, lungs, vagina, heart, or bloodstream (sepsis) (FDA, 2010).

Healthcare providers also prescribe metronidazole tablets to treat a sexually transmitted infection (STI) named trichomoniasis caused by tiny parasites. Once a laboratory has confirmed the organism, both sexual partners receive treatment simultaneously, even if only one has symptoms. 

Metronidazole gel is one of the most effective treatments for bacterial vaginosis, a common inflammation of the vagina. Vaginosis happens when there's a change in the number and types of bacteria in the vagina, and it's the most common cause of vaginal discharge in people. 

Healthcare providers can prescribe metronidazole "off-label" to treat other conditions. Off-label simply means the FDA didn't explicitly approve a drug for a particular use. A healthcare provider can nevertheless prescribe medications for an unapproved use if they decide that it's the proper treatment for their patient. 

Metronidazole has several off-label uses, including the treatment of bite wound infections, animal and human bites, Helicobacter pylori eradication, and tetanus (Weir, 2020).

It is important to note that metronidazole will not work for viral infections (i.e., common cold, flu, COVID-19). 

Like all medications, metronidazole can potentially cause adverse effects. Mild side effects tend to be more common, while severe side effects are relatively rare. 

The most common side effects are gastrointestinal, meaning they affect your stomach and intestines. Nausea, loss of appetite, vomiting, diarrhea, abdominal cramping, and constipation are common. You might also experience an unpleasant metallic taste after taking metronidazole which is nothing to worry about. It's important to avoid alcoholic beverages while taking metronidazole and not drink alcohol for at least three days after finishing the drug to avoid severe stomach upset/cramps, nausea, vomiting, headache, and flushing (FDA, 2010).

One thing you might notice while taking metronidazole is that your urine turns darker in color. This is generally no cause for concern and will go away after you finish the course of the medication.

In rare cases, metronidazole can cause nervous system effects like abnormal brain function, confusion, dizziness, and seizures. If you have any of these symptoms, contact your healthcare provider right away. 

When used as a gel, side effects are rare since very little metronidazole is absorbed systemically. Local effects like skin irritation or burning of the skin are uncommon.

Metronidazole can be administered orally (tablets and capsules), intravenously, or topically (creams or gels). 

When taken by mouth, take this medication with food or a full glass of water or milk to prevent stomach upset. The standard capsule dosing is 375 mg, but in the end, your healthcare provider will determine the correct dose based on the particular infection being treated. 

For bacterial vaginosis, your healthcare provider can prescribe metronidazole either as a pill—typically 500 mg twice daily for seven days—or a gel, applied inside the vagina at bedtime for five days. The choice of pill versus vaginal gel depends upon the patient's preference. There tend to be fewer side effects with the vaginal gel.

In general, it's important to continue metronidazole until the prescribed amount is finished, even if you feel better after a few days and symptoms have disappeared. Stopping the medication too early may result in a return of the infection.

Metronidazole comes with a black box warning that this drug could potentially cause cancer. While that sounds scary, it's important to keep in mind that this warning was based on animal studies in rats and mice, where cancer developed in some animals after they were given metronidazole. It's not clear if there is a similar risk for cancer in humans. The FDA says that "unnecessary use of the drug should be avoided" (FDA, 2010).

Metronidazole is considered a Pregnancy Category B medication, meaning that animal studies have not shown any risk to a fetus, and there are no adequate studies in humans (HHS, 2021). The consensus is that metronidazole should be used in pregnancy only if it is needed, and it should be avoided during the first trimester.

Drug interactions have the potential to change how your medications work and can increase your risk for serious side effects. While metronidazole cream or gel is not known to cause any problems with other medicines, oral and intravenous metronidazole can interact with other drugs. 

Most importantly, you should not use metronidazole if you recently drank alcohol or have taken disulfiram (Antabuse), a drug used to support the treatment of chronic alcoholism (FDA, 2010).

Some other products that may interact with this drug include:

• Alcohol-containing products (such as some cough and cold syrups)

• Lithium (mood stabilizer)

As with all medications, you should divulge any other drugs you're taking, follow your healthcare provider's instructions, and complete the entire course you've been prescribed. Should you have any side effects or concerns, speak with your provider to determine the best approach for you.

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Current version

June 04, 2021

Written by

Felix Gussone, MD

Fact checked by

Steve Silvestro, MD

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About the medical reviewer

Steve Silvestro, MD, Ro

Dr. Steve Silvestro is a board-certified pediatrician and Associate Director, Clinical Content & Education at Ro.