As vets, we have access to hundreds of medications to help with everything from sore ears to coughs, itchy skin to painful knees. One of the most common classes of drug that we use are antibiotics. Different conditions will require different antibiotics, but one of the more frequently prescribed tablets is metronidazole. Here, we will explain what metronidazole actually is and why it is usually prescribed. 

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Metronidazole as an antibiotic

Metronidazole is an antibiotic which is effective against anaerobic bacteria (those that can survive without oxygen), killing them by disrupting their DNA. In human medicine, it is used for some skin infections, dental abscesses, and pelvic inflammatory disease. In the veterinary world, metronidazole on its own is licensed for use in:

  • the treatment of gastrointestinal infections caused by Clostridia species.
  • the treatment of infections of the urogenital tract, oral cavity, throat, and skin caused by obligate anaerobic bacteria (e.g., Clostridia spp.) susceptible to metronidazole

It can also be combined with spiramycin to treat dental and other oral infections. 

Metronidazole as an anti-protozoal agent

Protozoa are tiny microorganisms which can be the cause of gastrointestinal infections in animals; especially the very young, very old, or immunocompromised. Although metronidazole has an indication for treatment of Giardia infections, its use has been superseded by drugs such as fenbendazole due to increasing levels of resistance. 

Common uses in veterinary practice

Most cases that receive metronidazole in first opinion practice, have the same presenting clinical sign – diarrhoea. Although metronidazole is indicated for gastrointestinal infections caused by Clostridia, this isn’t always tested for. And in many cases, it may not be implicated in actually causing the clinical signs. In those subjects where there is blood in the diarrhoea, the worry is that this means the intestinal wall could be compromised. But the only time when metronidazole has been shown to be of use in these cases, is when it leads to sepsis. In cases of chronic diarrhoea, where other therapies have not resolved the symptoms, metronidazole may be seen as a final trial therapy, and often will work, but why, when there is no obvious reason for it to?

Metronidazole’s secret weapon

Some antibiotics, including metronidazole, not only kill bacteria, but they also have anti-inflammatory and immunomodulatory actions in animal body tissues. In human medicine, dermatologists have been using this property of certain antibiotics to treat inflammatory skin conditions for years. And there are thoughts that it could prove useful in other inflammatory conditions throughout the body. Although the evidence is currently lacking, it is an area of research that could prove fruitful. 

There is also a small population of dogs with chronic diarrhoea that is colloquially known as ‘antibiotic responsive diarrhoea’. These cases are usually real cases of small intestinal bacterial overgrowth, or SIBO, in which metronidazole has been seen to be highly effective. 

Potential side effects of metronidazole

All drugs have potential side effects. These must always be weighed up against the benefits of the animal receiving the drug. With short term courses of metronidazole, side effects are rare; but because it has the potential to cause problems with the liver, it is not advisable to use it in animals with known liver disease. Vomiting and low white blood cell counts have also been reported. Prolonged courses, or high doses, of metronidazole can result in neurological signs though this is classed as ‘very rare’. 

Due to known toxic and potential (not proven) carcinogenic effects in humans, it is important that anyone handling the drug wears gloves. And it should not be handled at all by pregnant women. 

Conclusion

There are likely to be cases where metronidazole is prescribed when maybe it isn’t necessarily needed, but when used appropriately, it can be highly effective. In those specific infections highlighted at the start and in those cases of chronic diarrhoea, in which other common causes have been ruled out and there are signs of inflammation or as a trial for SIBO, metronidazole is a very useful and easy to dose drug. If you are unsure why your vet has prescribed metronidazole for your pet, or if you have concerns about its use or giving it, please do discuss it with them. 

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