Pharmacology of Metronidazole

Metronidazole is used to treat a wide variety of bacterial and protozoal infections. It was first introduced in France in 1960 and has, since then, gone on to become one of the most important antimicrobial medicines.

Metronidazole is used in the treatment of the following infections:

  • Clostridium difficile colitis
  • Gynecological infections – pelvic inflammatory disease, bacterial vaginosis
  • Oral /respiratory infections – periodontitis, aspiration pneumonia
  • Protozoal infections – giardiasis, trichomoniasis, amebiasis, dracunciliasis

Metronidazole is also used to treat other infections – eradication of H. pylori infections and to prevent infections in patients who have recently underwent surgery.

Mechanism of action

Metronidazole belongs to the nitroimidazole class. It inhibits nucleic acid synthesis and damages DNA.

However, metronidazole only works when converted to its reduced form – a nitroso free radical. Only anaerobic bacteria have the capacity to perform this reduction.

As a result, metronidazole is only effective at treating anaerobic bacteria and protozoa.

Side effects

Side effects associated with metronidazole include:

  • GI effects – nausea, vomiting
  • Hypersensitivity reactions – rash, itch, fever, flushing
  • Headache
  • Metallic taste
  • IV administration associated with thrombophlebitis
  • Topical administration linked to redness, dryness and skin irritation

When used for prolonged periods at high does, metronidazole is associated with neurological side effects such as optic and peripheral neuropathy, as well as hematological side effects such as leucopenia and neutropenia.

Other, rarer side effects include seizures and encephalopathy.

Clinical considerations

When we talk about the clinical pharmacology of metronidazole, we need to think about the following factors:

  • That dose should be reduced in patients with severe liver disease. This is because metronidazole is chiefly metabolised by the liver. It has an elimination half-life of approximately 8 hours. It is eliminated via the urine (80%) and feces (15%).
  • That metronidazole inhibits the enzyme responsible for metabolising alcohol – namely, acetaldehyde dehydrogenase. Taking metronidazole with alcohol, then, can lead to unpleasant side effects – nausea, flushing, headache and vomiting.
  • That metronidazole increases the risk of toxicity with lithium, warfarin and phenytoin.

Metronidazole remains an important medicine in the treatment of many bacterial and protozoal infections; a drug that has been around since 1960 and whose use is not likely to abate any time soon.

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