Metronidazole is available in several forms. Each form has different side effects.

Introduction

Metronidazole is a prescription drug used to treat infections caused by bacteria or other parasites in different parts of your body. It works by destroying the germs that cause infection. The drug is available in several forms:

  • immediate-release oral tablets and capsules
  • extended-release oral tablets
  • topical creams, gels, and lotions
  • vaginal gels

The different forms of metronidazole have different side effects. Read on to learn about the common and serious side effects that can occur with each form of metronidazole.

Metronidazole immediate-release and extended-release tablets and capsules are taken by mouth and are absorbed through the stomach. Extended-release drugs stay active in your body for a longer period of time than immediate-release drugs do.

The extended-release tablets are used to treat vaginal infections. The immediate-release forms of metronidazole are used to treat bacterial infections that affect many parts of the body as well as parasitic infections in the intestines, liver, and reproductive tract.

More common side effects

The most common side effects that can occur while taking metronidazole tablets or capsules include:

  • headache
  • loss of appetite
  • vomiting
  • diarrhea
  • heartburn
  • cramps in your abdomen
  • constipation
  • metallic taste in your mouth
  • yeast infection
  • vaginal discharge

Serious side effects

Serious side effects can occur while taking metronidazole tablets or capsules. These are all rare, however.

Cancer: Metronidazole tablets and capsules have a black box warning. This is the most serious warning from the Food and Drug Administration (FDA). In animal studies, cancer developed after animal subjects were given high doses of metronidazole for a long period. There may be a similar risk for cancer in humans.

Encephalopathy: In some cases, metronidazole tablets or capsules have caused encephalopathy (abnormal brain function). Symptoms can include:

  • a lack of muscle control for voluntary movements, such as walking or picking up objects
  • dizziness
  • slurred or slow speech that can be hard to understand

If you have any of these symptoms, call your doctor right away. Encephalopathy symptoms usually go away within days to weeks after stopping metronidazole.

Seizures: If you have a history of seizures, be sure to tell your doctor before starting metronidazole tablets or capsules. They may prescribe a different drug for you instead. And call your doctor right away if you have a seizure while taking metronidazole.

Meningitis: Meningitis is an inflammation of the membranes covering your brain and spinal cord. The swelling from meningitis can cause:

  • severe headache
  • fever
  • stiff neck

These symptoms can occur within hours of taking the drug. Meningitis can be life-threatening if not treated right away. Call your doctor immediately if you have any of these symptoms. The risk of meningitis typically goes away after treatment with metronidazole is stopped.

Peripheral neuropathy: Peripheral neuropathy is caused by damage to your nerves. Typically, the symptoms occur in your hands and feet. They can include:

  • a “pins and needles” sensation
  • a burning feeling
  • numbness
  • sharp, shooting pains

Call your doctor right away if you have any of these symptoms. Ongoing nerve damage can lead to muscle weakness, decreased reflexes, or loss of balance and coordination. Peripheral neuropathy may not go away after you stop taking metronidazole.

Severe allergic reaction: If you’re allergic to metronidazole, you may have symptoms such as:

  • trouble breathing
  • swelling of your tongue or throat
  • hives
  • rash

Sometimes, allergic reactions can be serious and require medical attention. If you have any of these side effects, stop taking metronidazole right away and call your doctor. If you have trouble breathing, call 911.

Stevens-Johnson syndrome: Stevens-Johnson syndrome is a rare but serious skin disorder. Symptoms can appear in this order:

  • flu-like symptoms, such as fever and tiredness
  • a painful red rash that spreads and blisters
  • shedding of the top layer of your skin

Stevens-Johnson syndrome is a medical emergency. If you have flu-like symptoms, call your doctor. If you have a painful rash that spreads and blisters or shedding of the top layer of your skin, call 911 or go to the nearest emergency room immediately.

Neutropenia: Neutropenia is the condition when you have low levels of certain white blood cells, called neutrophils, that help fight off infections. Neutropenia puts you at higher risk of infection. Call your doctor right away if you develop a new infection while taking metronidazole. Neutropenia will likely go away after you stop taking metronidazole tablets or capsules.

Metronidazole topical cream, lotion, and gel treat inflammation caused by rosacea. This condition causes the skin of your face to redden and form small bumps.

More common side effects

The more common side effects that can occur with metronidazole topical cream, gel, and lotion are generally mild. They may go away within a few days. If they’re more severe or don’t go away, talk to your doctor or pharmacist. These side effects can include yeast infection and the following skin effects:

  • burning and stinging
  • irritation
  • itching
  • worsened rosacea
  • dryness
  • redness

The topical gel may also cause these additional side effects:

  • common cold
  • upper respiratory tract infection
  • headache

Metronidazole vaginal gel treats bacterial vaginosis. This is a bacterial infection in the vagina.

More common side effects

The more common side effects that can occur with metronidazole vaginal gel are generally mild. They can include:

  • vaginal yeast infection
  • vaginal itching
  • headache
  • nausea
  • vomiting
  • menstrual cramps

Serious side effects

Serious side effects of the vaginal gel are rare. They include some of the same side effects that the tablets and capsules can cause, such as cancer, peripheral neuropathy, and seizures.

Do not drink alcohol or use any products that contain alcohol while using metronidazole. Doing so can cause a chemical reaction that prevents your body from processing the alcohol. Symptoms of such a reaction can include:

  • cramps in your abdomen
  • nausea
  • severe vomiting
  • headaches
  • flushing

Call your doctor if you have any of these side effects. And keep in mind that certain medications and other products can contain alcohol. These items can include:

  • cough syrups
  • mouthwashes
  • breath strips
  • aftershaves
  • perfumes
  • hair sprays
  • bug sprays
  • hand sanitizers
  • body washes

Check your labels for alcohol. You should avoid items with alcohol while using metronidazole and for three days after you finish treatment. Even a small amount of alcohol can cause a dangerous reaction.

If you have questions about metronidazole side effects, talk with your doctor. Your doctor can help you understand all of the effects of metronidazole. To get you started, try asking these questions:

  • Am I at higher risk for certain side effects from metronidazole?
  • Is there another medication I could take that might have fewer side effects?
  • Do you have suggestions for ways to prevent or reduce side effects?

Together, you and your doctor can help you treat your infection as safely as possible.