What is metronidazole for?

Metronidazole, better known in some homes as  Flagyl,  is a “multi-function” anti-infective drug. It is effective as an antibacterial and also as an antiparasitic. The FDA approved its use in 1963, initially as an agent against the parasite Trichomonas vaginalis, responsible for an unpleasant sexually transmitted disease called trichomoniasis . Mr. Trichomonas is a flagellated protozoan and perhaps it is not very crazy to think that this is the reason why they named the drug  Flagyl .


How does metronidazole work?

It has several mechanisms of action. Mainly the drug is able to sneak into the cells of the enemy microorganisms. Once inside, it is dedicated to producing free radicals, altering DNA and preventing the synthesis of nucleic acids and thus causing their death.


What “bugs” can metronidazole kill?

Although initially the drug was marketed with the aim of beating the Trichomonas parasite, soon afterwards they realized that it was also useful to combat other protozoan parasites such as Entamoeba histolytica and Giardia lamblia. And not happy with it, metronidazole had another ace up its sleeve that it had not yet discovered: it was also useful against anaerobic bacteria (those that live without oxygen), both gram-negative and positive. Among them we can highlight Helicobacter pylori as the protagonist. Come on, he’s not a “boy for everything”, but almost.

Metronidazole Indications

If a doctor has prescribed metronidazole, it is because you may have an infection that fits into one of these four blocks:

  • Urethritis and vaginitiscaused by trichomonas. It is used in both symptomatic and asymptomatic infections and in both men and women. It is important to distinguish vaginitis caused by trichomonas from vulvovaginitis caused by the fungus candida albicans. Candida and Trichomonas are mainly responsible for vulvovaginitis, which is the most common infectious process in the female underworld . It is mainly characterized by three symptoms: vulvovaginal pain, itching and burning, to which flow and dysuria (pain when peeing) are usually added. Differential diagnosis is important since candida vulvovaginitis is not treated with metronidazole but with clotrimazole .
  • Lambliasis: Giardia lamblia is another flagellated protozoan. This one, instead of vaginas and urethras, where he likes to spend his days is in the intestine. It is a relatively frequent parasitosis in children that must be treated because it can cause malabsorption of nutrients and weight loss. A Cochrane review published in 2012 indicates that albendazole can be similarly effective as metronidazole, with the advantage of fewer side effects and a simplified regimen.
  • Amebiasis: It is also used in acute intestinal amebiasis (amoebic dysentery) and in amebic liver abscess caused by Entamoeba histolytica.
  • Anaerobic bacterial infections. Like those caused by the bacteria  Bacteroides fragilis and Clostridium perfringes.

 What are the metronidazole presentations?

  • Ovules or vaginal tablets: Their trade name is Flagyl and, due to their format, they are those used in urethritis and trichomonas vaginitis. Its use should always be combined with oral administration of metronidazole (metronidazole 250 tablets). The dosage and administration regimen can be consulted in the technical file .
  • Oral tablets: It is used for the four blocks mentioned above and the dosage and regimen will vary depending on the age and characteristics of each patient. They can be consulted in its  technical file .
  • Suspension: It is also used for the four blocks mentioned above, provided that due to the characteristics of the patient, the pharmaceutical form in suspension is more suitable (for example, for children who are treated for giardiasis).
  • Topical metronidazole. Their trade names are Rozex or Metronidazol Viñas 0.75% . and its price is identical (€ 5, 37). They have a different indication: rosacea. They are used as the recommended topical starting treatment, in patients with a limited number of papules or pustules, for the relief of inflammatory lesions.

If I have trichomoniasis, does my partner also have to be treated?

Watch the couples! Whether or not the couple shows clinical signs of Trichomonas vaginalis infection (even if the laboratory result is negative), it is necessary to treat it at the same time with 250 mg of oral metronidazole.

Can “resistance” to metronidazole be produced?

Yes, and although metronidazole is the primary drug of choice for the treatment of trichomoniasis, the presence of resistant strains from many different countries highlights the need for new drugs for treatment.

Can metronidazole cause cancer?

When the results of studies showing linkage between metronidazole and cancer in the mouse and rat were published, the alarms went off. However, similar studies in the hamster have had negative results and epidemiological studies in man have not shown any evidence of increased cancer risk in men. However, for all these reasons, the use of metronidazole in prolonged treatment should be carefully considered and, of course, never be used without a prescription.

Can metronidazole be used in pregnancy?

Metronidazole crosses the placental barrier and rapidly penetrates the fetal circulation. He does not recommend using metronidazole during the first trimester of pregnancy for the treatment of trichomoniasis. Its use during the second and third trimester to treat trichomoniasis should be limited to patients whose symptoms are not controlled with local palliative treatment.

Can metronidazole be used in lactation?

In this case of lactation there is some controversy. On the one hand, the technical sheet, revised in November 2016, indicates that it is recommended to interrupt breastfeeding, and it can be resumed 24 to 48 hours after completing the treatment. On the other hand, the e-lactation website classifies it as “very low” risk and compatible with breastfeeding. It is therefore essential to consult with the doctor who prescribes the treatment.



by Abdullah Sam
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