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Flagyl is an agent against anaerobic microorganisms (Peptostreptococcus, Clostridium sp., Bacteroides sp., Prevotella, Fusobacterium, Veillonella), the protozoa (Trichomonas vaginalis, Giardia intestinalis, Entamoeba histolytica).

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Flagyl (metronidazole) is an antibiotic that is used to treat a wide variety of infections.

How does it work?

Metronidazole is a nitro-5-imidazole. The mechanism of action of metronidazole is the biochemical reduction of the 5-nitro group of metronidazole by intracellular transport proteins of anaerobic microorganisms and protozoa. The reduced 5-nitro group of metronidazole interacts with the DNA of the cell of microorganisms, inhibits the synthesis of their nucleic acids, which kills microorganisms. The main metabolite (2-oxymetronidazole) also has antiprotozoal and antimicrobial effects.

What bacteria kills Flagyl?

The antibiotic is active against Trichomonas vaginalis, Entamoeba histolytica, as well as gram-negative anaerobes Bacteroides spp. (including B. fragilis, B. ovatus, B. distasonis, B. thetaiotaomicron, B. vulgatus), Fusobacterium spp., and some gram-positive anaerobes (sensitive strains of Eubacterium spp., Clostridium spp., Peptococcus niger, Peptostreptococcus spp.). The MIC for these strains is 0.125-6.25 μg / ml. In combination with amoxicillin, it is active against Helicobacter pylori (amoxicillin inhibits the development of resistance to metronidazole).

Aerobic microorganisms and facultative anaerobes are insensitive to metronidazole, but in the presence of mixed flora (aerobes and anaerobes), metronidazole acts synergistically with antibiotics effective against conventional aerobes.

When is Flagyl used?

The antibiotic is used to treat infectious diseases caused by microorganisms Bifidobacterium, Bacteroides fragilis, Bilophilia, Clostridium perfringens, Clostridium difficile, Eubacterium, Peptostreptococcus, Fusobacterium, Prophyromonas, Prevotella and Veilonella.

The drug is effective in the treatment of amebiasis, nonspecific vaginitis, urogenital trichomoniasis, surgical infections, giardiasis.

Tablets can be prescribed as a continuation of non-oral metronidazole therapy.


Flagyl is not used in patients who have allergic reactions to metronidazole, its derivatives and to any component of the drug.

The drug is not used in patients who have a sensitivity to gluten, as well as in patients with complex diseases of the central nervous system (there is a high probability of deterioration of the condition).

The drug is not used to treat children weighing less than 16 kg (usually children under 6 years).

Flagyl is not used together with disulfiram and/or ethyl alcohol.

Used with caution: patients with blood diseases (including those cured). During the course of treatment, the patient’s blood condition should be constantly monitored.

How to take Flagyl?

The drug is taken orally.

Doses and duration of treatment are determined by the doctor, based on the general picture of the course of the disease, the tolerance of the drug by the patient, additional drugs that the patient takes.

Amoebiasis is usually treated with 500 mg three times a day for a week.

For children, the dose is calculated based on body weight (the daily dose is 30-40 mg per kilogram of weight). The drug is taken three times a day.

Giardiasis usually treated with 375-500 mg for 5 days. “Children’s” dose is 250 mg.

Trichomoniasis (vaginitis or urethritis) is treated in “children’s” doses with the same frequency of administration for 10 days.


An overdose of metronidazole causes vomiting, ataxia and disorientation.

There is no specific antidote. Usually, an overdose is treated with standard techniques for intoxication.

Side effects

Flagyl is usually well tolerated. But n rare cases, the drug can cause side effects:

  • sensory peripheral neuropathy;
  • headache, dizziness;
  • ataxia;
  • convulsions;
  • hallucinations;
  • confusion of consciousness.

The digestive system can respond with nausea, vomiting, stool disturbances, pain, dry mouth, and stomatitis.

There is also a decrease in vision, allergic reactions, a change in the color of urine.

All cases have a reversible consequence and go away on their own.

If you have atoxia, blurred consciousness and dizziness, you’d better cancel the drug. If any of the side effects occur, you should inform your doctor.

Pregnancy and breastfeeding

The influence of Flagyl on pregnancy is not fully understood. Therefore, it is recommended to prescribe the drug carefully with regular and careful monitoring of the condition of the mother and fetus. If there is a safer and more researched alternative to this antibiotic, you’d better apply it.

The antibiotic tends to penetrate into the amniotic fluid and breast milk, therefore it is recommended either to refrain from feeding during the treatment period or to stop breastfeeding.

Drug interactions

  • Similarly to disulfiram, Flagyl causes intolerance to ethanol;
  • Simultaneous use with disulfiram can cause various neurological symptoms (the interval between uses is at least 2 weeks);
  • Metronidazole strengthens the action of indirect anticoagulants, which leads to an increase in the time of prothrombin formation;
  • Cimetidine inhibits the metabolism of metronidazole, which can lead to an increase in its concentration in the blood serum and an increased risk of side effects;
  • The simultaneous administration of drugs that stimulate enzymes of microsomal oxidation in the liver (phenobarbital, phenytoin) can accelerate the elimination of metronidazole, as a result of which its concentration in plasma decreases.

Special instructions

The use of the drug with alcohol can cause a feeling of fever, vomiting, tachycardia, During treatment with Flagyl, patients should not consume alcoholic beverages or medications containing alcohol.

Long-term use of the drug is allowed under the control of the blood count. With leukopenia, the possibility of continuing treatment depends on the risk of developing an infectious process.

When treating Trichomonas vaginitis in women and Trichomonas urethritis in men, it is necessary to refrain from sexual activity. Simultaneous treatment of sexual partners is required. Treatment does not stop during your menstruation. After trichomoniasis therapy, control tests should be carried out within 3 successive cycles before and after menstruation.

If symptoms persist 3-4 weeks after treatment, conduct 3 fecal analyzes at intervals of several days.

Treatment should be discontinued if the patient has ataxia, dizziness, hallucinations, and the neurological status of patient worsens.

Metronidazole can immobilize treponemas, which leads to a false positive Nelson’s test.

Since the antibiotic may cause dizziness and other side effects, it is recommended to refrain from driving and other mechanisms.