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Flagyl Side Effects and What You Can Do About Them

Medically reviewed by Leigh Ann Anderson, PharmD. Last updated on July 14, 2022.

Flagyl: Decades of Bad Side Effects

The US Food and Drug Administration (FDA) first approved Flagyl (generic name: metronidazole) way back in the 1960's, so you've probably taken it, or know someone who has.

It's a potent drug in the nitroimidazole class used to treat bacterial infections of the:

  • stomach and intestine
  • skin
  • joints
  • vagina
  • respiratory tract.

Metronidazole is bactericidal (kills bacteria) and is also used to treat protozaol infections like Trichomoniasis, Giardiasis or Amebiasis. It's used to treat ominous sounding organisms such as Bacteriodes fragilis, Helicobacter pylori, and Giardia lamblia.

Flagyl is an important drug in the world of medicine, but it's linked with many difficult side effects. So how can you better tolerate this medicine to cure what is most likely a serious infection?

Strengths and Uses: Metronidazole

Your metronidazole (Flagyl) dose and length of therapy will vary depending upon your infection. It's often prescribed to be taken by mouth two or three times a day for 7 to 14 days; it depends upon the type of infection you have.

Oral metronidazole comes in three oral strengths:

  • 250 and 500 mg strength, regular-release tablets
  • 375 mg regular release capsules.

Metronidazole is used to treat many common infections like:

  • bacterial vaginosis
  • bacterial stomach or intestinal infections
  • pelvic inflammatory diseases
  • trichomoniasis (sexually transmitted infection due to a parasite).

It's also available in an intravenous (IV) formulation for injection, and as a cream or lotion for use in acne treatment or rosacea, and in a topical gel for vaginal infections (bacterial vaginosis).

How to Handle the Stomach Side Effects of Flagyl

Metronidazole side effects are notorious for causing an upset stomach and killing your appetite. For many people, the nausea -- and even vomiting -- can occur just from the bad taste.

Heartburn, constipation, and diarrhea can occur, too. It's enough to make some people abandon therapy altogether, but don't -- call your doctor before you stop any antibiotic.

The regular release tablets can be taken with a meal, a snack or a glass of milk to help prevent upset stomach.

If heartburn is an issue, ask your doctor if you can take an over-the-counter acid blocker like famotidine (Pepcid AC).

What Can I Do About the Flagyl Taste?

Metronidazole (brand: Flagyl) leaves a bitter, metallic taste in your mouth. This is a well-known side effect. For many, the bad taste lasts throughout treatment, too, not just when you're swallowing the medicine.

How can you mask the bad tase of metronidazole?

  • During treatment, have a supply of sweet hard candies (sugar-free, if preferred) or mints to help mask the taste.
  • If the bitter taste is only difficult when you're swallowing your medicine, try masking with chocolate, like chocolate pudding or chocolate milk.
  • If you can't tolerate the taste at all, or if you have nausea or vomiting, ask your doctor if there are other options to treat your infection.

Flagyl and Alcohol: Never Shaken or Stirred

The metronidazxole (Flagyl) pharmacy bottle sticker says "Avoid Alcohol", but how strict is this?

It's important; in fact, you should not have ANY alcohol with metronidazole, and that includes, beer, wine, spirits and liquors in mixed drinks. You should even avoid alcohol in medicines (like cough or cold syrups) or mouthwash while you are taking metronidazole and for at least 3 days after you stop taking it. Also avoid propylene glycol, a synthetic food additive that's in the alcohol group, too.

What are the effects if you don't follow this rule? Unpleasant side effects can occur, such as:

  • a fast heartbeat
  • warmth or redness (flushing) under your skin
  • a tingly feeling
  • nausea and vomiting reportedly may occur.

Although there are only a few case reports, the manufacturer still recommends alcohol and propylene glycol avoidance with both systemic (oral, IV) and topical forms of metronidazole. And let's face it -- metronidazole is hard enough on its own without compounding the side effects with alcohol, don't you agree?

Antibiotic-Induced Diarrhea: It's Unfair

It's bad enough you have an infection. But now your treatment has caused antibiotic-associated diarrhea.

  • This happens because the population of "good" bacteria (normal microflora) in your intestine are altered due to your antibiotic.
  • Symptoms can range from soft stools to frequent and explosive watery diarrhea.

Is there anything you can do? Many people eat yogurt with live, active cultures or take a probiotic to help prevent this problem while taking antibiotics.

Be sure to stay hydrated, replace electrolytes, and eat a bland diet like bananas, rice, applesauce, and toast (BRAT diet) to help recover from a bout of diarrhea. Antibiotic-associated diarrhea, while unpleasant, is usually a short-lived side effect and should clear up a few days after antibiotics are stopped. But see the next slide for a warning.

Clostridium Difficile: A More Severe Form of Diarrhea

A more severe case of infectious diarrhea called Clostridium difficile (C. difficile or "C. dif") can happen with almost any antibiotic, and is more common when hospitalized. Serious complications can include inflammation of the intestine known as pseudomembranous colitis.

However, the good news is that metronidazole is used to treat C. difficile, so you should have some coverage for this bacteria.

Here's an important point to remember: if you have severe diarrhea that is watery, bloody, or contains mucus; stomach cramping or fever, call your doctor right away. Also, avoid any over-the-counter antidiarrheal medicine unless your doctor recommends it.

Your Brain On Flagyl

Feel like your world is spinning? Having trouble focusing? Your metronidazole treatment might be to blame. Flagyl can penetrate the blood-brain barrier and is known to cause central nervous system (CNS) side effects that can worsen with high doses. Common ones include:

  • Irritability, dizziness, or confusion
  • Difficulty sleeping
  • Headaches
  • Tremors or shakiness
  • Peripheral neuropathy (tingling or painful sensations in the hands or feet)
  • Seizures
If you experience any serious nervous system effects, or ones you simply can't tolerate, call your doctor right away. Don't drive, operate machinery, or do anything requiring mental alertness until you know how this medicine affects you.

Kill the Bacteria: Get a Yeast Infection

Antibiotics can frequently lead to unpleasant yeast (candida) infections. Women may get an itchy, vaginal yeast infection. Antibiotics kill off the “good” bacteria as well as bacteria causing your infection, which alters the balance of yeast in the vagina.

Although a yeast infection is annoying and can be uncomfortable, it's easy to treat. Visit your pharmacy for 7-day, 3-day or even 1-day vaginal suppository or cream treatment options.

Common brands include Monistat (miconazole) or Gyne-Lotrimin (clotrimazole)

  • If these treatments aren't effective, see your doctor for a higher strength cream, different antifungal or an oral treatment.
  • Plus, if this is your first yeast infection after a course of antibiotics, check with your doctor to be sure it's not a more serious problem.
  • And don't get confused; even though metronidazole ends in "azole", it will not treat any fungal or yeast infection.

Changes in Urine Color

It can be shocking to see your urine color change to a dark or reddish-brown color when taking metronidazole (Flagyl).

But never fear, this pigment change is due to a metabolite of metronidazole that occurs when it's broken down in your body for elimination through your kidneys, so it's nothing to be concerned about.

In fact, many other medications -- including rifampin, sulfasalazine, and propofol -- can change the color of urine; it's not an uncommon effect.

Your urine should change back to it's normal color a day or two after you stop treatment.

Flagyl's Too Hard to Tolerate: Can't I Just Stop It?

Be sure to take metronidazole for the full course of antibiotic treatment that your doctor prescribes. The side effects may be somewhat difficult, but a recurring infection if you stop therapy can be serious.

If you simply cannot tolerate the side effects, speak with your health care provider about what other options you might have. Also, don't ever share this medication with anyone - it's very specific for the type of infection it treats and may be harmful if taken by someone else without a doctor's okay. It has no action on viral infections like a cough, cold, the flu, or COVID.

What are your thoughts on metronidazole?

  • If you took metronidazole (Flagyl), were you able to tolerate it?
  • Which side effects did you experience?
  • Would you take metronidazole again, if recommended by your doctor?

Consider joining the Metronidazole Support Group if you have more questions, answers or opinions.

Finished: Flagyl Side Effects and What You Can Do About Them

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  • Hempel S, Newberry SJ, Maher AR, et al. Probiotics for the prevention and treatment of antibiotic-associated diarrhea: a systematic review and meta-analysis. JAMA 2012; 307:1959–1669. Accessed July 14, 2022 at DOI: 10.1001/jama.2012.3507
  • Kusumi RK, Plouffe JF, Wyatt RH, Fass RJ. Central nervous system toxicity associated with metronidazole therapy. Ann Intern Med 1980; 93:59. Accessed July 14, 2022 at DOI: 10.7326/0003-4819-93-1-59
  • Flagyl (metronidazole) monograph. Accessed July 14, 2022 at
  • Johnson M et al. Metronidazole: An Overview. Up to Date. Accessed May 7, 2021.

Further information

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.