Fidaxomicin
Macrolides
Macrolides
Oral macrocyclic antibiotic that is hardly absorbed and acts in the gut against Clostridioides difficile, with few relapses.
Quick facts
- Group
- Macrolides
- Target
- Bacterial RNA polymerase (Clostridioides difficile)
- Used for
- Clostridioides difficile diarrhoea
- Key effects
- Nausea and vomiting · Abdominal pain · Constipation
- Monitor
- Number and look of stools · Signs of allergy
- ATC
- A07AA12
No doses: the clinician sets them by the current guideline.
Mechanism of action
The figure shows Clostridioides difficile RNA polymerase with fidaxomicin bound.
Fidaxomicin binds bacterial RNA polymerase and stops transcription from starting, so the bacterium makes no RNA. It is bactericidal against C. difficile and largely spares the normal gut flora. It barely reaches the blood, so it only works for infection of the colon.
- Taken by mouth, it stays in the gut lumen
- Binds C. difficile RNA polymerase
- Blocks the start of transcription
- Kills the bacterium and spares much of the flora
Indications
- Clostridioides difficile diarrhoea, including the first episode and relapses Clostridioides difficile infection →
Adverse effects
Common
- Nausea and vomiting
- Abdominal pain
- Constipation
Contraindications
- Hypersensitivity to fidaxomicin or its ingredients
Key interactions
- Ciclosporin and other P-glycoprotein inhibitors
- Raise fidaxomicin blood levels, which stay very low; treatment is not changed.
What is monitored
- Number and look of stools
- Signs of allergy, above all with a history of macrolide allergy
- Signs of relapse after treatment ends
Administration and safety
- Oral, with or without food; a suspension also exists and is kept refrigerated
- Does not treat infections outside the gut: it never replaces a systemic antibiotic
Pregnancy and breastfeeding
Data in pregnancy are not enough to know the risk; animal studies showed no fetal harm.
Nursing care
- Ask about macrolide allergy
- Reinforce hand washing with soap and water and contact precautions
- Teach to finish the course
- Ask the patient to report diarrhoea that comes back after treatment
Pharmacokinetics
- Absorption
- Minimal systemic absorption; blood levels are very low.
- Metabolism
- Hydrolysed to an active metabolite (OP-1118), without cytochrome P450.
- Elimination
- In the faeces: more than 92% of the dose is recovered there.
- Half-life
- About 12 hours for the drug and its metabolite (healthy adults).
Test yourself
Think of the answer, then tap the question to see it.
Which enzyme does fidaxomicin act on?
Bacterial RNA polymerase: it stops transcription from starting in C. difficile.
Why is fidaxomicin useless for pneumonia?
It is hardly absorbed; it stays in the gut lumen and does not reach useful tissue levels.
What advantage does fidaxomicin have over oral vancomycin for C. difficile?
It spares more of the normal gut flora and is linked to fewer relapses.
Which allergy history calls for closer watch in someone receiving fidaxomicin?
Macrolide allergy: some people who reacted to fidaxomicin had it.
Connections
← Back to the class: Macrolides
Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.
Sources: DailyMed - FIDAXOMICIN tablet, coated (DailyMed (U.S. National Library of Medicine), 2026); Fidaxomicin: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2026); Clostridioides difficile RNAP with fidaxomicin (RCSB Protein Data Bank, 2026).
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Representative model, for educational purposes only.