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How do I know if metronidazole is working?

Metronidazole is a common antibiotic used to treat a variety of bacterial and parasitic infections. It goes by the brand names Flagyl, Metro, or Protostat. When taking metronidazole, it is normal to wonder if it is working effectively to treat your infection. Here are some signs that can help you determine if metronidazole is working for you.

Symptoms Improve

The most obvious sign that metronidazole is working is improvement in the symptoms that prompted you to take the antibiotic in the first place. For example, if you have bacterial vaginosis and are experiencing discharge and odor, you should notice these symptoms start to resolve within a few days of starting metronidazole. Improvement in symptoms indicates the antibiotic is effectively killing the microbes causing the infection.

Some common infections and symptoms that should improve with metronidazole treatment include:

  • Bacterial vaginosis: vaginal discharge, odor, irritation
  • Dental infections: tooth pain, swelling
  • STDs like trichomoniasis: vaginal discharge, itching, burning
  • H. pylori infection: stomach pain, nausea
  • Skin infections: redness, swelling, pain
  • Bacterial diarrhea: watery, frequent stools

Keep in mind it can take a few days for metronidazole to start working, so you may not notice immediate improvement. But within 3-7 days you should see a reduction in symptoms if the antibiotic is effective.

Finish the Full Course

It is important to finish the entire course of metronidazole, even if you start feeling better. This helps ensure the infection has been completely treated. Stopping the antibiotic too soon can allow bacteria to survive and become resistant to metronidazole.

A typical course of metronidazole lasts 7-10 days. In some cases, like H. pylori infection, you may need to take the antibiotic for 10-14 days along with other medications. Always finish all pills prescribed by your doctor.

If symptoms have not sufficiently improved by the time you complete the course of metronidazole, this likely means it was ineffective and a different antibiotic may be needed. Let your doctor know if this is the case.

Test Results Improve

For some bacterial infections, your doctor may retest after a course of antibiotics to confirm the infection has been eradicated. Examples include:

  • H. pylori: breath test or stool antigen test
  • Bacterial vaginosis: vaginal culture
  • Dental infections: dental exam
  • STDs like trich or chlamydia: urine test or vaginal swab

If a test comes back negative after antibiotic treatment, this signals the infection is gone and metronidazole worked successfully.

Side Effects Resolve

Metronidazole causes side effects in some people, especially when first starting treatment. Common side effects include:

  • Nausea or vomiting
  • Metallic taste in mouth
  • Loss of appetite
  • Diarrhea
  • Abdominal pain
  • Yeast infection
  • Urine darkening

These side effects should gradually improve and disappear a few days after finishing metronidazole. If you still have persistent nausea, stomach pain, or other concerning reactions lasting more than a few days post-treatment, contact your doctor.

Symptoms Don’t Return

If symptoms of infection return shortly after finishing metronidazole, this suggests the antibiotic did not fully eliminate the bacteria. Some reasons this may occur include:

  • Not completing the full course of medication
  • Reinfection from a sexual partner
  • Resistant bacteria not killed by metronidazole

Recurrence of symptoms usually means a different antibiotic or longer treatment course is required. Let your doctor know if symptoms come back after metronidazole.

When is Metronidazole Not Working?

There are a few signs that indicate metronidazole is not effectively treating an infection:

  • No improvement in symptoms
  • Tests remain positive for infection after treatment
  • Symptoms return shortly after finishing antibiotics

Reasons metronidazole may fail to work include:

  • Incorrect diagnosis – infection is not bacterial
  • Resistant bacteria
  • Abscessed tooth or other source of bacteria not reached by antibiotic
  • Impaired immune system unable to clear infection
  • Non-compliance taking the medication

Your doctor can determine if metronidazole is not working for you and make adjustments to your treatment plan accordingly. Don’t hesitate to follow up if your symptoms are not resolving as expected.

How Metronidazole Works

To understand how to determine if metronidazole is effectively treating an infection, it helps to understand how the medication works:

  • Metronidazole is an antibiotic that kills bacteria and parasites. It is effective against both aerobic and anaerobic microorganisms.
  • It disrupts the DNA of microbial cells, preventing them from replicating and spreading infection.
  • Metronidazole is rapidly absorbed and distributed to tissues after oral administration.
  • It is metabolized by the liver and excreted by the kidneys.
  • Typical course is 7-10 days depending on type of infection.
  • Common side effects include nausea, metallic taste, and urine darkening.

By killing the microbes causing an infection, metronidazole eliminates symptoms and allows the body to heal. Noticeable improvement in symptoms is a good indicator the antibiotic is working effectively.

Tips for Metronidazole Effectiveness

Here are some tips to help ensure metronidazole works optimally:

  • Take it consistently: Take metronidazole at regular intervals around the clock to maintain steady antibiotic levels.
  • Don’t drink alcohol: Alcohol inhibits the liver’s ability to metabolize metronidazole and can cause severe reactions.
  • Finish the course: Take metronidazole for the full prescribed duration to completely eliminate bacteria.
  • Monitor symptoms: Note symptom improvement to confirm the antibiotic is working.
  • Get tested: Have follow-up lab tests done to prove infection has resolved.
  • See your doctor: Follow up with your doctor if symptoms persist after treatment.

Properly taking metronidazole as prescribed and closely monitoring your response to treatment can help optimize its effectiveness against infection.

When to Call Your Doctor

Consult your doctor if you experience:

  • No improvement in symptoms within a few days of starting metronidazole
  • Worsening symptoms or new symptoms develop
  • Severe nausea, vomiting, diarrhea, stomach pain
  • Neurological symptoms like tingling hands/feet, dizziness, confusion
  • Symptoms return shortly after finishing antibiotic course

Rarely, metronidazole can cause a severe allergic reaction or neurotoxicity. Seek immediate medical care if you have trouble breathing, hives, swelling, or sudden weakness.

Your doctor can assess your condition and adjust treatment if metronidazole appears ineffective or is not tolerated.

Common Bacterial Infections Treated with Metronidazole

Metronidazole is commonly prescribed for the following bacterial infections:

Bacterial Vaginosis

Bacterial vaginosis is a polymicrobial imbalance in the vagina involving overgrowth of anaerobic bacteria like Gardnerella vaginalis. Symptoms include discharge, odor, burning, and itching. Metronidazole is a first-line antibiotic treatment, taken orally or vaginally for 5-7 days.


Trichomoniasis is a sexually transmitted infection caused by the parasite Trichomonas vaginalis. Symptoms include vaginal discharge, burning, and itching. Metronidazole is the preferred antibiotic treatment, taken orally in a single 2g dose or a 7-day course.


Giardiasis is infection of the small intestine caused by the parasite Giardia lamblia, acquired through contaminated food or water. Symptoms include diarrhea, gas, nausea, and abdominal cramps. A 7-10 day course of metronidazole is standard treatment.


Amebiasis is intestinal infection caused by the parasite Entamoeba histolytica and spread by contaminated food and water. Symptoms include diarrhea, abdominal pain, fever, and bloody stools. Metronidazole is used for intestinal infection while liver abscesses require aspiration.

H. Pylori Infection

H. pylori is a bacterial infection of the stomach lining and major cause of gastritis, peptic ulcers, and stomach cancer. Treatment involves a triple therapy regime of metronidazole, a proton pump inhibitor, and either amoxicillin or clarithromycin for 10-14 days.

Periodontal Disease

Periodontal disease is bacterial infection of the gums leading to inflammation, bleeding, and bone loss around teeth. Metronidazole combined with amoxicillin may be used alongside dental deep cleaning for severe gum infection treatment.


Diverticulitis is infection or inflammation of intestinal pouches called diverticula. Symptoms include fever, nausea, vomiting, and abdominal pain. Mild diverticulitis is often treated with oral antibiotics like metronidazole for 7-10 days.

Bacterial Skin Infections

Bacterial skin infections encompass various conditions like cellulitis, impetigo, and secondary infections of wounds. Topical metronidazole gel or oral forms may be used depending on severity and location.

Bacterial Vaginosis Treatment Guidelines

Bacterial vaginosis is a dysbiosis of the vaginal bacteria, characterized by reduced lactobacilli and overgrowth of anaerobic bacteria. Diagnosis is made based on Amsel criteria:

  • Thin grayish-white discharge
  • Fishy odor upon adding alkali
  • Clue cells on microscopy
  • Vaginal pH > 4.5

2015 CDC treatment guidelines include:

  • Metronidazole 500 mg orally 2x day for 7 days
  • Alternatives: metronidazole gel 0.75% daily for 5 days OR clindamycin cream 2% daily for 7 days
  • Treat partner(s) with oral metronidazole or tinidazole 2g single dose
  • Resistance is increasing with metronidazole, up to 5-7% of cases
  • Recurring BV may require longer metronidazole regimen, up to 14 days

Regular condom use, vaginal probiotics, and hydrogen peroxide baths may help prevent recurrence of bacterial vaginosis after antibiotic treatment.

When is Metronidazole Prescribed?

Metronidazole is primarily prescribed for bacterial vaginosis, trichomoniasis, H. pylori infection, and protozoal infections like giardiasis and amebiasis. It may also be used to treat dental, respiratory, skin, and surgical infections caused by susceptible anaerobic bacteria.

Key indications for prescription of oral or intravenous metronidazole include:

  • Bacterial vaginosis
  • Trichomoniasis
  • Giardiasis
  • Amebiasis
  • H. pylori infection
  • Dental and gum infections
  • Diverticulitis
  • Brain abscess
  • Septicemia
  • Joint infections
  • Diabetic foot infections
  • Secondary bacterial infections

Metronidazole provides coverage against anaerobes not covered by other common antibiotics like penicillins and cephalosporins. Resistance remains low among most obligate anaerobes.

Dosage Information for Oral Metronidazole

Metronidazole is available in oral forms including tablets, capsules, and suspensions:

  • Tablets: 250mg, 500mg
  • Extended-release tablets: 750mg
  • Capsules: 375mg
  • Suspension: 200mg/5mL

Typical oral metronidazole dosing for adults is:

  • 250-500mg 3 times daily for 7-10 days
  • 2g single dose for trichomoniasis
  • Up to 500-750mg 3 times daily for 14 days for severe infections

Pediatric dosing of metronidazole is:

  • 30-50mg/kg/day divided every 6-8 hours
  • Maximum daily dose is 2g

Metronidazole dosing may need adjusted in kidney failure. The IV form is given by infusion for serious infections.

How Long Does it Take Metronidazole to Work?

Metronidazole starts working against bacteria and parasites relatively quickly. Here is the typical timetable for metronidazole effectiveness:

  • 30-90 minutes: Metronidazole is absorbed and reaches maximum blood plasma concentrations
  • 6 hours: Diffuses into tissues and body fluids to reach sites of infection
  • 24 hours: Initial improvement in symptoms may be noticed
  • 2-3 days: Many patients experience substantial improvement in symptoms
  • 5-7 days: Course completed, most patients are symptom-free

The full antimicrobial effects take time to develop, but people generally start to feel better within a day or two after starting metronidazole treatment.

How Long do Metronidazole Side Effects Last?

The most common side effects of metronidazole like nausea and metallic taste occur shortly after starting treatment and last for the first few days. Here is the typical duration of common metronidazole side effects:

  • Nausea – Starts within hours of the first dose, lasts 2-6 days
  • Vomiting – Can begin within hours, resolves 1-3 days
  • Metallic taste – Starts 1-2 days into treatment, lasts 3-10 days
  • Loss of appetite – Develops over 2-3 days, resolves after treatment ends
  • Diarrhea – Can occur in first