Side Effects of Flagyl (Metronidazole)

This Antibiotic Is Used To Treat Infections And Pouchitis

Young woman sitting on bed with headache
Flagyl can cause several troublesome side effects such as headache and dry mouth.. Tetra Images/Brand X Pictures/Getty Images

What Is Flagyl?

Flagyl is a type of antibiotic and an antiprotozoal medication that is used to treat bacterial infections in the body. The generic name for this antibiotic is metronidazole, and it also may be sold as protostar. Flagyl may be used to treat infections occurring in the skin, reproductive organs, and digestive tract. It is often taken as a capsule, but it can also be given through an IV.

Flagyl is sometimes used to treat Crohn's disease, which is one form of inflammatory bowel disease (IBD), especially when the disease is affecting the perianal area or just the colon. It may also be used to treat pouchitis, which is a poorly-understood condition that occurs in people who have had surgery to remove their large intestine and create an internal pouch called a j-pouch.

Flagyl is also used to treat an infection in the digestive tract with a bacteria called Clostridium difficile. C difficile infections are becoming increasingly prevalent, and are especially dangerous for people with IBD. For treating pouchitis and C difficile, Flagyl is sometimes taken for long periods of time, and perhaps along with another antibiotic, or alternating with another antibiotic.

Flagyl is also used in combination with other drugs to treat an infection with Helicobacter pylori. H pylori infects the stomach and can lead to stomach ulcers.

Flagyl And Alcohol

It is extremely important that people who take Flagyl not drink alcoholic beverages until at least 3 days after stopping the drug. Flagyl affects the way alcohol is broken down, and that leads to symptoms of nausea and vomiting. 

Other Important Considerations

It's not common, but some people experience a tingling in their hands and feet when taking Flagyl.

If that happens, stop taking the drug immediately and call the doctor to figure out what to do next. All antibiotics can reduce the effectiveness of oral contraceptives ("the pill"), so women using this as their birth control method should use a backup method. Flagyl can also sometimes cause urine to turn dark brown, but this is not dangerous and it goes away after the drug is stopped.

Dangerous side effects aside, Flagyl, and all other antibiotics, should always be taken until they are finished. Stopping antibiotics early could lead to even more dangerous strains of bacteria. If you have concerns about this or any other drug, check with your physician.

Common Side Effects of Flagyl

Check with your doctor if any of the following side effects continue or are bothersome:

  • diarrhea
  • dizziness or lightheadedness
  • headache
  • loss of appetite
  • nausea or vomiting
  • stomach pain or cramps

Less Frequent or Rare

Check with your doctor if any of the following side effects continue or are bothersome:

  • change in taste sensation
  • dryness of mouth
  • unpleasant or sharp metallic taste

Check With Your Doctor

Less common

  • any vaginal irritation, discharge, or dryness not present before use of this medicine
  • clumsiness or unsteadiness
  • mood or other mental changes
  • skin rash, hives, redness, or itching
  • sore throat and fever
  • stomach and back pain (severe)

For injection form

  • Pain, tenderness, redness, or swelling over vein in which the medicine is given

Notify Doctor Immediately

Less common

  • numbness, tingling, pain, or weakness in hands or feet

Rare

  • convulsions (seizures)

Other Notes

Metronidazole (Flagyl) may cause dark urine in some patients. This is temporary and will go away when you stop taking this medicine.

Other side effects not listed above may also occur in some patients. If you notice any other effects, check with your doctor. This information is meant only as a guideline - always consult a physician or pharmacist for complete information about prescription medications.

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