Pharmacology for Technicians PTCB Test Prep

PTCB Guide to Metronidazole!

Sep 2nd, 2020
metronidazole pharmacology

What is “metronidazole”?

Metronidazole is one of the most prescribed antimicrobial medicines; a drug used in the treatment of a wide range of infections. Here, in our PTCB guide to metronidazole, we tease out the details that you need to know.

If you are preparing for the PTCB exam, you may already have come across metronidazole.

You may remember that metronidazole is used to treat a range of bacterial and protozoal infections, and that it is the active ingredient of medicines such as Flagyl and Metrogel.

You may even know the medicine because it has been prescribed in the past to you, or to someone you know. With more than 12 million prescriptions per annum, metronidazole is the 58th most prescribed medicine in the United States. Against these statistics, it is likely you have already heard of the medicine.

The remarkable effectiveness of metronidazole lies in its mechanism of action, which later we explore in more detail. For now, though, let’s take a moment to review some of the key indications of this essential medicine.

Indications of Metronidazole

Metronidazole is used to treat the following bacterial infections:

  • Antibiotic-associated colitis – inflammation of inner colon lining
  • Oral / respiratory infections – dental abscesses, aspiration pneumonia
  • Pelvic inflammatory disease
  • Helicobacter pylori infection – alongside a penicillin and a macrolide

Metronidazole is also used to treat a variety of protozoal infections, including:

  • Trichomoniasis
  • Giardiasis
  • Amoebic dysentery

Metronidazole may also be used to treat other infections not listed in this guide.

Mechanism of Action

In terms of the pharmacology of metronidazole, the drug works – at the basic level – by binding to the DNA of organisms. This disrupts the process of DNA synthesis and causes degradation and death of the cell.

Metronidazole achieves this effect in anaerobic bacteria (bacteria that do not live or grow when oxygen is present) and protozoa – where the drug is metabolized to form a nitroso free radical. It is this radical that binds to DNA to cause the subsequent disruption to DNA synthesis and, ultimately, cell death.

Aerobic bacteria do not have the capacity to reduce metronidazole to this nitroso free radical, hence why metronidazole cannot be used to treat aerobic bacterial infections.

Side Effects

Common side effects associated with metronidazole include:

  • Metallic taste
  • Nausea and vomiting
  • Hypersensitivity reactions
  • Brown-discolored urine

At high doses – or if the drug is taken for a long time – metronidazole can cause central nervous system effects such as optic neuropathy and seizures.

If taken with alcohol, metronidazole causes a “disulfiram-like” reaction.

Disulfiram is a drug used to treat alcoholism – where, if taken with alcohol, causes hangover-like effects (such as nausea) to happen there and then. Therefore, it deters patients from drinking alcohol.

In the case of metronidazole, it has a “disulfiram-like” reaction when taken with alcohol – where patients may feel nauseous or have abdominal pain or general discomfort.

Drug Interactions

For the PTCB exam, students should also know some of the common drug interactions with warfarin.

These include:

  • Metronidazole increases the bleeding risk of warfarin (anticoagulant)
  • Metronidazole increases the risk of phenytoin toxicity (anticonvulsant)
  • Metronidazole increases the risk of toxicity with lithium (bipolar medicine)

Because metronidazole is metabolized by the liver, patients with active liver disease require a lower dose. Similarly, older patients may require a lower dose due to reduced metabolism.

Study Review

Remember, the new PTCB syllabus assigns a weighting of 40% to the Medications knowledge domain.

Metronidazole is one of the most widely prescribed antibacterial drugs and so students should have a thorough knowledge of this medicine. If you have taken practice PTCB exam questions, you may have come across questions such as:

  • What is metronidazole used to treat?
  • What is the active ingredient of the medicine, Flagyl?
  • Which medicine causes a “disulfiram-like” reaction with alcohol?
  • What is the active ingredient of the medicine, Metrogel?
  • Metronidazole increases the bleeding risk of which medicine?

If you know the answers to these questions, you can be sure to have this topic covered for the day of your PTCB test.

Check back to PTCB Test Prep soon for more guides on antibacterial drugs; practice test questions; and the must-know facts to help you ace the exam!

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PTCB Test Prep Author


Elaine Walker

Elaine joined PTCB Test Prep in 2017, currently serving as the lead product development manager overseeing both course development and quality improvement. Mrs. Walker is a graduate of California State University and has worked as a pharmacy technician for over twenty years – with particular interests in pediatric pharmacy, extemporaneous compounding, and hospital pharmacy. Over the past 8-years, she has helped prepare thousands of students for the PTCB examination.