Flagyl Side Effects

Frequency not reported: Dysarthria, hypoesthesia, paresthesia, peripheral sensory neuropathy, transient epileptiform seizures, hearing impaired/hearing loss (including sensorineural), tinnitus, taste disorders/unpleasant taste in mouth, nystagmus, convulsive seizure, numbness, syncope, vertigo, paralysis [Ref]

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?

The per capita price of healthcare per year is higher in the United States than in any other nation in the world, according to National Public Radio (NPR). America spends nearly 2.5 times as much per person as the United Kingdom does, despite having comparable wealth and a lower life expectancy.

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 taste 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

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 Drugs.com 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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Sources

• 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. Drugs.com. Accessed July 14, 2022 at https://www.drugs.com/flagyl.html
• 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.

Flagyl Side Effects

Medically reviewed by Drugs.com. Last updated on Mar 3, 2023.

Note: This document contains side effect information about metronidazole. Some dosage forms listed on this page may not apply to the brand name Flagyl.

Applies to metronidazole: oral capsule, oral powder for suspension, oral tablet. Other dosage forms:

Warning

Oral route (Tablet; Tablet, Extended Release)

Metronidazole has been shown to be carcinogenic in mice and rats. Unnecessary use of the drug should be avoided. Its use should be reserved only for conditions for which it is approved.

Serious side effects of Flagyl

Along with its needed effects, metronidazole (the active ingredient contained in Flagyl) may cause some unwanted effects. Although not all of these side effects may occur, if they do occur they may need medical attention.

Check with your doctor immediately if any of the following side effects occur while taking metronidazole:

More common

• Agitation
• back pain
• blindness
• blurred vision
• burning, numbness, tingling, or painful sensations in the hands or feet
• changes in speech patterns
• confusion
• decreased vision
• depression
• dizziness
• drowsiness
• eye pain
• fever
• headache
• irritability
• lack of coordination
• nausea
• seeing or hearing things that are not there
• seizures
• shakiness and unsteady walk
• slurred speech
• stiff neck or back
• trouble speaking
• unsteadiness, trembling, or other problems with muscle control or coordination
• unusual tiredness or weakness
• vomiting
• weakness in the arms, hands, legs, or feet

Less common

• Black, tarry stools
• blood in the urine
• body aches or pain
• burning while urinating
• chills
• clumsiness or unsteadiness
• difficulty with breathing
• ear congestion
• fainting
• feeling of pelvic pressure
• frequent or painful urination
• irregular heartbeat
• loss of voice
• nasal congestion
• pinpoint red spots on the skin
• runny nose
• skin rash, hives, redness, itching
• sneezing
• stomach and back pain (severe)
• unusual bleeding or bruising
• vaginal irritation, discharge, or dryness not present before taking the medicine

Rare

• Bleeding gums
• bloating
• chest pain
• constipation
• cough
• dark urine
• fast heartbeat
• indigestion
• loss of appetite
• painful or difficult urination
• pains in the stomach, side, or abdomen, possibly radiating to the back
• sore throat
• sores, ulcers, or white spots on the lips or in the mouth
• swollen glands
• yellow eyes or skin

Incidence not known

• Blistering, peeling, or loosening of the skin
• bloody or cloudy urine
• burning while urinating
• continuing diarrhea
• continuing stomach pain
• diarrhea
• feeling of warmth
• increased volume of pale, dilute urine
• joint or muscle pain
• loss of bladder control
• red skin lesions, often with a purple center
• red, irritated eyes
• redness of the face, neck, arms, and occasionally, upper chest
• redness of the skin

Other side effects of Flagyl

Some side effects of metronidazole may occur that usually do not need medical attention. These side effects may go away during treatment as your body adjusts to the medicine. Also, your health care professional may be able to tell you about ways to prevent or reduce some of these side effects.

Check with your health care professional if any of the following side effects continue or are bothersome or if you have any questions about them:

More common

• Dizziness or lightheadedness
• feeling of constant movement of self or surroundings
• heartburn
• sensation of spinning
• stomach cramps
• trouble sleeping
• weight loss

Rare

• Change in taste sensation
• congestion
• dry mouth
• pain or tenderness around the eyes and cheekbones
• tender, swollen glands in the neck
• trouble with swallowing
• unpleasant or sharp metallic taste
• voice changes

Incidence not known

• Decreased interest in sexual intercourse
• inability to have or keep an erection
• loss in sexual ability, desire, drive, or performance
• painful sexual intercourse

For Healthcare Professionals

Applies to metronidazole: compounding powder, intravenous powder for injection, intravenous solution, oral capsule, oral suspension, oral tablet, oral tablet extended release.

General

The most serious side effects reported were convulsive seizures, encephalopathy, aseptic meningitis, optic neuropathy, and peripheral neuropathy (characterized by numbness/paresthesia of an extremity). [Ref]

Nervous system

Very common (10% or more): Headache (up to 18%)

Common (1% to 10%): Dysgeusia/taste perversion (e.g., metallic taste, sharp unpleasant metallic taste, taste of alcoholic beverages modified), dizziness

Rare (0.01% to 0.1%): Encephalopathy (e.g., confusion, fever, headache, hallucinations, paralysis, light sensitivity, disturbances in sight and movement, stiff neck), aseptic meningitis, seizure, somnolence, peripheral neuropathy, ataxia

Very rare (less than 0.01%): Subacute cerebellar syndrome (e.g., ataxia, dysarthria, gait impaired, nystagmus, tremor), drowsiness, convulsions, incoordination

Frequency not reported: Dysarthria, hypoesthesia, paresthesia, peripheral sensory neuropathy, transient epileptiform seizures, hearing impaired/hearing loss (including sensorineural), tinnitus, taste disorders/unpleasant taste in mouth, nystagmus, convulsive seizure, numbness, syncope, vertigo, paralysis [Ref]

Gastrointestinal

Very common (10% or more): Nausea (up to 10%)

Common (1% to 10%): Abdominal pain, diarrhea, dry mouth, stomatitis, glossitis

Rare (0.01% to 0.1%): Pancreatitis, upper abdominal pain/epigastralgia, vomiting

Frequency not reported: Constipation, tongue discoloration, oral mucositis, furry/furred tongue, gastrointestinal disturbances (e.g., epigastric pain/distress), proctitis, sudden overgrowth of Candida in mouth, abdominal cramping, dyspepsia, pseudomembranous colitis, mucositis [Ref]

Genitourinary

Very common (10% or more): Vaginitis (up to 15%)

Common (1% to 10%): Genital pruritus, abnormal urine, dysmenorrhea, moniliasis, urinary tract infection

Rare (0.01% to 0.1%): Chromaturia

Very rare (less than 0.01%): Darkened urine

Frequency not reported: Vaginal dryness, vulva dryness, dysuria, vaginal candidiasis, dyspareunia, polyuria, incontinence, cystitis, pyuria, proliferation of Candida in vagina [Ref]

Other

Uncommon (0.1% to 1%): Asthenia

Rare (0.01% to 0.1%): Mucosal inflammation, pyrexia/fever

Very rare (less than 0.01%): Flushing

Frequency not reported: Malaise, face edema, peripheral edema, chest pain, chills, overgrowth of Candida, disulfiram-like reaction, weakness, sensation of pelvic pressure [Ref]

Respiratory

Frequency not reported: Dyspnea, nasal congestion, hiccup [Ref]

Musculoskeletal

Common (1% to 10%): Myalgia

Very rare (less than 0.01%): Arthralgia

Frequency not reported: Muscle spasms, fleeting joint pain, stiff neck [Ref]

Hematologic

Uncommon (0.1% to 1%): Leukopenia

Frequency not reported: Eosinophilia, bone marrow aplasia, bone marrow depression, blood dyscrasia [Ref]

Dermatologic

Very rare (less than 0.01%): Skin rash, pustular eruptions, acute generalized exanthematous pustulosis, pruritus

Frequency not reported: Fixed drug eruption, swelling face, urticaria, hyperhidrosis, erythema, drug reaction with eosinophilia and systemic symptoms (DRESS), erythematosus rash, mild erythematous eruptions, pustulosis [Ref]

Hepatic

Rare (0.01% to 0.1%): Cholestatic hepatitis

Very rare (less than 0.01%): Increased liver enzymes (AST, ALT, alkaline phosphatase), cholestatic/mixed hepatitis, hepatocellular liver injury, jaundice, liver failure requiring liver transplant, abnormal liver function tests

Frequency not reported: Increased liver enzyme, hepatotoxicity/liver failure in patients with Cockayne syndrome, drug-induced hepatitis [Ref]

Hypersensitivity

Rare (0.01% to 0.1%): Anaphylaxis, anaphylactic shock, Jarisch-Herxheimer reaction

Frequency not reported: Serum sickness-like reaction, immediate/delayed hypersensitivity reactions, anaphylactic reaction, hypersensitivity

Psychiatric

Rare (0.01% to 0.1%): Hallucinations

Very rare (less than 0.01%): Confusion/confusional state, psychotic disorders

Frequency not reported: Depression, insomnia, depressed mood, decreased libido, irritability, psychosis, disorientation, psychotic reaction [Ref]

Ocular

Rare (0.01% to 0.1%): Optic neuropathy, diplopia, myopia

Very rare (less than 0.01%): Vision disorders (e.g., diplopia, myopia)

Frequency not reported: Optic neuropathy/neuritis, saccadic eye movement, blurred vision, decreased visual acuity, changed color vision, light sensitivity [Ref]

Cardiovascular

Frequency not reported: Flattening of T-wave on ECG, tachycardia, palpitations, QT interval prolonged on ECG [Ref]

Metabolic

Frequency not reported: Anorexia, decreased appetite [Ref]

Oncologic

An increased incidence of gastrointestinal and certain extraintestinal cancers have been reported in patients with Crohn’s disease. Breast and colon cancer have been reported in medical literature in Crohn’s disease patient treated with high doses of this drug for a prolonged duration; causality was not established. [Ref]

Frequency not reported: Breast cancer, colon cancer, gastrointestinal cancer, certain extraintestinal cancers [Ref]

Local

Frequency not reported: Injection site reaction, thrombophlebitis [Ref]

Frequently asked questions

• What happens if you drink alcohol with metronidazole?
• How long does it take for metronidazole to work?
• How long does it take metronidazole to leave your system?
• Can metronidazole cause a yeast infection?
• Does metronidazole treat chlamydia?
• Can you have sex while taking metronidazole?
• Can you use Soolantra and metronidazole together?

More about Flagyl (metronidazole)

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