Metronidazole side effects and how to avoid them

Metronidazole is an antibiotic used to stop the growth of some bacterial and parasitic infections. Metronidazole is sold under the brand names Flagyl and Flagyl ER.

By Lindsay Modglin | Apr. 26, 2021 Medically reviewed by Stephanie Melby, Pharm.D.

Metronidazole is an antibiotic used to stop the growth of some bacterial infections and parasitic infections including skin infections, bacterial vaginosis (BV), trichomoniasis, and pelvic inflammatory disease (PID). It also may be used to treat an abscess. Metronidazole is sold under the brand names Flagyl and Flagyl ER (extended-release). It may come as an oral medication, vaginal gel, cream, or lotion. There is also an IV form of metronidazole that some healthcare professionals will use in a hospital setting.

There are several common side effects including headache, cramping, and nausea. In general, there are fewer side effects with topical and vaginal treatment options. Drug interactions are also possible when taking metronidazole, so it’s important to understand the risks before beginning treatment.

Common side effects of oral metronidazole

The most common side effects of metronidazole tablets and capsules include:

Nausea and vomiting
Headache
Loss of appetite
Diarrhea
Stomach pain or cramping
Lightheadedness or dizziness
Constipation
Dry mouth
Metallic taste in the mouth
Skin rashes
Inflammation of the mouth, lips, or tongue
Dark urine (red/brown color)
Yeast infection

Nausea and diarrhea

Nausea and diarrhea are the most common side effects of metronidazole. According to the Federal Drug Administration, about 12% of patients taking metronidazole tablets report experiencing nausea, which is sometimes accompanied by diarrhea. Metronidazole tablets can be taken with food such as a meal or snack to help reduce stomach upset. However, the extended-release version of metronidazole must be taken on an empty stomach at least one hour before eating or at least two hours after eating.

If intestinal side effects are a concern, people can be prescribed a topical or vaginal form of metronidazole if appropriate for the type of infection. These forms of metronidazole have less chance of causing nausea and diarrhea.

Common side effects of topical and vaginal metronidazole

The most common side effects of metronidazole vaginal gel, cream, and lotion include:

Skin irritation including mild dryness or burning sensation
Itching
Nausea
Metallic taste in the mouth
Headache (vaginal gel only)
Vaginal discharge (vaginal gel only)
Yeast infection (vaginal gel only)

Yeast infections

Metronidazole should not be used to treat a yeast infection (candidiasis). In fact, the use of metronidazole may cause a yeast infection in some. It’s estimated that 10% of women taking metronidazole end up with a yeast infection as a side effect of treatment.

While metronidazole is effective in getting rid of the bad bacteria that causes an infection, it may also destroy helpful bacteria that normally live in the vagina. This can lead to a disruption in the environment of the vagina and an overgrowth of yeast.

For women who often get yeast infections when using antibiotics, their doctor may proactively prescribe them an antifungal medicine such as Diflucan (fluconazole).

Serious side effects of oral metronidazole

Serious side effects of metronidazole tablets and capsules include:

Seizures
Meningitis
Peripheral neuropathy
Damage to the optic nerve
Brain damage
Anaphylaxis (severe allergic reaction producing hives and swelling of the face and throat)
Stevens-Johnson syndrome (medication-induced syndrome causing flu-like symptoms and painful rash)
Toxic epidermal necrolysis (rare but serious skin condition causing blistering and peeling)
Low white blood cells
Low platelets
Neutropenia (low neutrophils)
Pancreatitis (inflammation of the pancreas)
Prolonged QT interval (disturbances in heart activity especially if taken with other medications that prolong the QT interval)

How long do metronidazole side effects last?

The most common side effects of metronidazole will improve as soon as treatment is finished. Depending on the type of infection, people may use metronidazole for only one day or up to 14 days. If diarrhea or severe vomiting occurs, a healthcare provider may switch a person to a different medication.

Some severe side effects may require a person to discontinue treatment with metronidazole. Allergic reactions with metronidazole are rare, but still possible. People should contact their doctor and discontinue use immediately if they experience any signs of a more severe reaction including hives, trouble breathing, severe itchiness, or swelling of the face or throat.

Metronidazole contraindications & warnings

Metronidazole may not be the right treatment for everyone. Some people may be more sensitive to metronidazole, making other drug options a better fit. If people have had an allergic reaction while taking metronidazole in the past, they should make sure to tell a healthcare provider. They should discuss taking a different antibiotic to avoid another reaction.

Fortunately, metronidazole does not create dependency. High doses of the drug may cause more severe nausea and vomiting, as well as a lack of muscle coordination (ataxia). If overdose is expected, people should call their local poison control or go to the emergency room for evaluation.

Liver disease

If someone has liver disease, metronidazole may not be the best treatment option. Because metronidazole is processed in the liver, it can lead to an increased risk of toxicity in the liver. Despite this risk, people with liver disease may still able to safely take metronidazole. Healthcare providers will be able to determine if their patient should avoid taking metronidazole.

Seizures

Metronidazole is one of the medications that may lower the seizure threshold. This means a person is more likely to have a seizure when taking this drug. People with a history of seizures or epilepsy should get medical advice from their healthcare provider before taking this drug. If treatment with this drug is still necessary, extra seizure precautions should be put in place.

CNS disorders

Metronidazole may cause peripheral neuropathy. This means that a person may experience weakness, numbness, or pain in the hands or feet. People with CNS (central nervous system) disorders should use caution when taking this medication. Watch for signs of ataxia including slurred speech, difficulty walking, falling, or other coordination issues.

Blood disorders

A severe adverse effect of metronidazole includes low blood cell counts. For this reason, patients with blood cell abnormalities may need to be monitored closely during treatment with metronidazole. Healthcare providers may want to perform a simple set of lab work before starting treatment.

Cockayne syndrome

People with Cockayne syndrome, a rare disease causing premature aging, an abnormally small head, and growth and learning delays, should not take metronidazole. The drug may cause an increased risk for liver failure. However, if healthcare providers believe metronidazole is necessary, they may want to check liver function before and after treatment. Watch for the following signs of liver damage while taking this medication:

Yellowing of the skin or eyes (jaundice)
Severe right-sided abdominal pain
Abdominal swelling
Nausea and/or vomiting

Metronidazole while pregnant

Metronidazole is an FDA pregnancy category B drug and is approved for use during pregnancy. This means that animal studies show no harm to the fetus, but human studies are still needed to determine risk. There is limited data to show a risk for birth defects or miscarriage when taking metronidazole during pregnancy.

Metronidazole while breastfeeding

Although metronidazole can be prescribed while breastfeeding, it may travel into the breast milk. It could cause diarrhea and/or oral thrush in the infant. If a larger dose of metronidazole is prescribed, a mother should consider pumping and dumping her milk throughout the duration of therapy plus an additional 24 hours after stopping metronidazole.

Metronidazole and children

Metronidazole is approved for use in children. Children may take metronidazole for some infections including amebiasis, trichomoniasis, C difficile infection (C diff.), or H. pylori. If a child has a history of kidney or liver problems, the healthcare provider may decide to cut the dose in half.

Metronidazole interactions

There are a few potential drug interactions when taking metronidazole. People should make sure to tell their doctor what medications they are taking before beginning treatment.

Some potential interactions include:

Disulfiram: Disulfiram, a drug used for recovering alcoholics, can react with metronidazole to cause a psychotic reaction. This serious reaction can include confusion, hallucinations, or delusions. Avoid taking disulfiram for at least two weeks before starting metronidazole.
Blood thinners: Use caution when taking metronidazole with blood thinners, like warfarin. Taking these medications together may cause a person to bleed more easily. A person’s PT and INR may need to be checked before and during treatment so that the warfarin dose can be adjusted accordingly.
Lithium: Metronidazole may interact with the way lithium is removed from the body, leading to potential toxic effects.
Busulfan: Metronidazole should be avoided with the chemotherapy treatment, busulfan. Metronidazole can increase the amount of busulfan in the body and lead to toxic amounts.
Alcohol: People should not drink alcohol when taking metronidazole. This interaction causes severe nausea, vomiting, flushing, dizziness, headache, and general symptoms of being unwell. In addition to alcoholic beverages, people need to avoid other products that may contain alcohol. Some medications and food products contain alcohol or propylene glycol. The ingredient propylene glycol is a synthetic food additive that is grouped in the alcohol family, so it is best to avoid this as well. It’s important to discuss any current over-the-counter and prescription drugs with a doctor before taking metronidazole.

How to avoid metronidazole side effects

Metronidazole may not be right for everyone, so it’s important to discuss the medication with a healthcare provider before beginning treatment. Some side effects may make treatment with metronidazole uncomfortable, but there are ways to prevent or reduce these side effects and make treatment more tolerable.

Some ways to prevent adverse reactions when taking metronidazole include:

1. Take metronidazole as directed

People should follow their healthcare provider’s directions when taking metronidazole. The dosing and treatment depend on what infection is being treated. Since metronidazole is available in a variety of formulations, a doctor can prescribe a topical or vaginal formulation to help lessen side effects if appropriate for the type of infection. Healthcare providers may prescribe a dose once to a few times a day, or a single one-time dose. It’s important to read through the drug information provided by the pharmacy when picking up medication.

2. Take metronidazole with food

Oral metronidazole may cause an upset stomach, or even vomiting. Side effects tend to be worse when taking metronidazole tablets or capsules on an empty stomach. Unless the metronidazole is extended-release, pairing doses with a meal or snack will help prevent these adverse reactions from occurring or becoming severe.

3. Avoid alcohol and alcohol-containing products when taking metronidazole

Alcohol and metronidazole should never be mixed because they can cause increased side effects. People should refrain from drinking alcohol during treatment and for three days after finishing treatment to ensure they won’t develop adverse reactions. A few medications that contain alcohol include Nyquil, Robitussin, and Vicks Cough. To find out if an over-the-counter medication contains alcohol, read the ingredients label on the medication bottle or box.

4. Finish the entire treatment regimen

Don’t stop taking metronidazole before treatment is finished, even if your symptoms have disappeared. It is common for people to start feeling better early on in their course of therapy, so it’s important to remember to still complete the entire course of treatment. Not completing an antibiotic treatment may cause resistance, in turn, making the drug less effective. It’s important to take metronidazole exactly how a healthcare provider prescribed it.

5. Spread doses evenly throughout the day

Doses of metronidazole should be spread evenly throughout the day when multiple doses are prescribed. If people miss a dose, they should take the missed dose as soon as they remember. It’s important to not take two doses at the same time. If people have trouble remembering to take their medication, they may need to set a timer as a reminder or use a pillbox for the course of their treatment.

6. Talk to a doctor about all medications before beginning treatment with metronidazole

Some medications may interact with metronidazole, so it’s important for people to discuss any prescriptions they are taking with their doctor, as well as over-the-counter and herbal supplements. A doctor will determine if there is a risk for drug interactions during treatment. If an interaction is possible, a doctor may decide to treat with an alternative drug.

7. Avoid sexual intercourse if you are taking metronidazole for trichomoniasis

Trichomoniasis is a sexually transmitted infection, meaning that people became infected as the result of having sexual intercourse with someone who has it. Metronidazole is a common medication used to treat this infection. If someone is being treated for trichomoniasis, their partner(s) may also need to be treated. This prevents people from becoming reinfected after treatment is finished. Avoid having sexual intercourse until finishing the entire course of antibiotics and have been given approval by a healthcare professional. It’s also important to avoid sexual intercourse when using the vaginal gel form of metronidazole.

7. Take a probiotic

Using a probiotic while taking antibiotic therapy such as oral metronidazole can help reduce side effects such as stomach upset and diarrhea. Probiotics can help supplement the intestines with good bacteria that may have been damaged with an antibiotic. It’s important to separate an antibiotic dose from a probiotic dose by at least 2 hours. Probiotics are available to purchase at pharmacies and grocery stores.