Medical Fun Facts Podcast

MFF0020: Flagyl®

So what can I choose that begins with F? How about Faecal Microbiota Transfer? How about Fungi? How about Flagyl®.

So tonight I chose Flagyl®. That’s the trade name. The generic name is metronidazole.

Metronidazole is one of those really cool medicines that can have different uses. It’s not just an antibacterial agent but it also works on some parasites. It also has some important side effects that users need to be aware of.

Metronidazole can work on procaryotic and eucaryotic cells. It acts by inhibiting the synthesis of DNA. It can only do so when metronidazole is chemically reduced. That is when metronidazole gains or accepts electrons. This occurs commonly in anaerobic environments which means metronidazole has activity against bacteria and parasites in anaerobic conditions with little effect on human cells and aerobic bacteria. That doesn’t mean that it will only kill bacteria in deep-seated places though, it will kill lots of good bacteria living in our gut. Every time a doctor prescribes you an antimicrobial, just remember that antimicrobial doesn’t have a precise targeting system. You may have an infection in your lung, but most antimicrobials as they’re absorbed will have bacteria-killing activity all across your body. We should use antimicrobials carefully.

So what is metronidazole used for in conventional medicine?

On surgical wards, you often hear the holy trinity of antimicrobials being prescribed when a patient may have an acute abdomen. You want something that will prevent peritonitis and cover Gram-positive and Gram-negative aerobic bacteria as well as anaerobic bacteria. The holy trinity of antimicrobials is ampicillin, gentamicin and metronidazole. The holy trinity is cheap as chips and even today it has its uses. Unfortunately, in some settings, it is not so useful because of our poor history of inappropriate and overuse of antimicrobials. I hear a lot of medical practitioners complain about agriculture and primary industry, but dare I say, he who is without sin may cast the first stone.

Think about a lovely abscess full of bacteria and cellular debris. The chemical reactions going on in a large walled off loculated abscess like an ischiorectal abscess tends towards chemical reduction and an anaerobic environment. After a surgeon has done her or his work by incising the abscess and deloculating it with her or his finger to allow all that lovely pus to drain out, metronidazole will often be prescribed to ‘mop’ up the anaerobic bacteria.

There are other uses for metronidazole and they include the treatment of:

The important side effects include:

Some common ones like nausea and diarrhoea plus a headache, abdominal pain and a metallic taste in your mouth.

Then there are the serious ones like a reduction in some white blood cells, damage to peripheral nerves and central nervous system toxicity, especially in patients who are administered high doses for long periods of time.

Metronidazole is also a potential human carcinogen.

One thing you don’t want to do while taking metronidazole is drink alcohol. It is thought that metronidazole has a disulfiram-like reaction that can make you feel very unwell with nausea, vomiting, skin flushing, shortness of breath and a racing heart. There is some debate on the exact mechanism, but even so, it’s advisable not to have the two together.

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