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Antibiotic-associated diarrhoea, from mild upset to C difficile - a stepped UK approach.

Loose stools during or after antibiotics are common. Most are mild and self-limiting, but a smaller group have Clostridioides difficile infection that needs prompt testing and treatment.

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Why trust this guide

  • 01

    Editorial team

    Written by our editorial team from the UK sources listed on each page.

  • 02

    Sourced from guidance

    Checked against NICE NG199, UK Health Security Agency and peer-reviewed sources you can see at the end.

  • 03

    Current for 2026

    Reflects modern UK practice including fidaxomicin as a first-line option, bezlotoxumab and NICE TA1071 on faecal microbiota transplant.

Key facts

AAD and C difficile at a glance.

The essentials, in plain English: what it is, which antibiotics carry the greatest risk, and how it is treated in the UK today.

  • What it is

    Diarrhoea occurring during or up to 8 weeks after antibiotic exposure, ranging from a mild self-limiting upset to life-threatening Clostridioides difficile colitis.

  • How common

    About 5 to 25 per cent of patients receiving antibiotics develop diarrhoea; only a subset is due to C difficile.

  • Highest-risk drugs

    Clindamycin, third and fourth generation cephalosporins, fluoroquinolones and co-amoxiclav carry the greatest risk. Tetracyclines and PPIs alone carry the least.

  • Two broad types

    Non-infectious osmotic or motility diarrhoea (mild, self-limiting) and Clostridioides difficile infection (CDI), which can be severe.

  • CDI hallmark

    Watery diarrhoea with fever, abdominal pain, raised white cell count and, at the extreme, pseudomembranous colitis or toxic megacolon.

  • Foundation therapy

    Stop the offending antibiotic when possible, rehydrate and, for confirmed CDI, treat with oral vancomycin or fidaxomicin per NICE NG199.

Why this guide matters

Test early, treat properly, prevent spread.

C difficile is under-recognised in the community and over-treated with the wrong drugs in some hospital settings. Three points shape the rest of this page.

  • Not every loose stool is CDI

    Most antibiotic-associated diarrhoea is mild, osmotic and self-limiting. Testing focuses on those with severity features or CDI risk factors.

  • Fidaxomicin has changed first-line

    NICE NG199 recognises oral fidaxomicin alongside oral vancomycin as first-line treatment for mild to moderate CDI, favouring it where recurrence matters.

  • Prevention is the biggest win

    Antibiotic stewardship, soap-and-water hand hygiene and side-room isolation stop outbreaks faster than any single drug.

How the diagnosis is made

From first loose stool to a confirmed plan.

The steps a UK GP or hospital team will normally follow, in order, so you know what to expect and why.

  1. 01

    Assessing

    Timing and drug history

    Onset of loose stools during or within 8 weeks of an antibiotic course, especially clindamycin, a cephalosporin, a fluoroquinolone or co-amoxiclav.

  2. 02

    Assessing

    Clinical assessment

    Stool frequency, fever, abdominal pain, dehydration and any features of ileus or toxic megacolon such as abdominal distension.

  3. 03

    Assessing

    Risk factors for CDI

    Recent hospital stay, age over 65, proton pump inhibitor use, immunosuppression, chemotherapy or previous CDI raise the pre-test probability.

  4. 04

    Confirming

    Two-step stool testing

    UK pathway: GDH (glutamate dehydrogenase) screen first, then toxin A/B enzyme immunoassay or PCR to distinguish infection from colonisation.

  5. 05

    Confirming

    Severity bloods

    White cell count, creatinine, albumin and lactate stratify CDI into mild, moderate, severe or fulminant disease.

  6. 06

    Confirming

    Rule out other pathogens

    Consider Norovirus, Salmonella, Shigella, Campylobacter, enterohaemorrhagic E coli, Cryptosporidium and Giardia when the picture is atypical.

  7. 07

    Escalating

    CT abdomen if fulminant

    Imaging is reserved for suspected toxic megacolon, perforation or ileus and guides urgent surgical review.

Typical timeline: stool testing within hours, treatment decisions within a day.

Symptoms

What AAD and CDI look like.

From loose stools alone to fever, systemic upset and colitis - and the features that mean it is time to escalate.

  • Loose or watery stools

    Three or more unformed stools in 24 hours during or after antibiotics is the working definition.

  • Cramping abdominal pain

    Colicky lower-abdominal pain, sometimes with bloating and urgency, is common in CDI.

  • Fever and malaise

    Fever above 38.5 C and systemic upset point toward CDI rather than a simple osmotic diarrhoea.

  • Blood or mucus in stool

    Frank blood is unusual; mucus and pseudomembranes on endoscopy suggest CDI colitis.

  • Dehydration

    Thirst, reduced urine output, tachycardia and postural dizziness signal a need for fluid replacement.

  • Raised white cell count

    A WCC above 15 x 10 9/L helps define severe CDI and prompts inpatient care.

  • Abdominal distension

    A distended, tender abdomen with reduced bowel sounds may indicate ileus or evolving toxic megacolon.

  • Red flag - fulminant CDI

    Hypotension, shock, ileus, megacolon or lactate above 5 mmol/L is a surgical emergency.

Treatment

How AAD and CDI are treated in the UK.

Stop the trigger, replace fluids and, for confirmed CDI, treat with oral fidaxomicin or vancomycin. For recurrent disease, bezlotoxumab and FMT are now options.

  • Stop the offending antibiotic

    Where clinically safe, discontinue or narrow the antibiotic that triggered the diarrhoea. This alone resolves many mild cases.

  • Rehydration and electrolytes

    Oral rehydration for mild disease; intravenous fluids and potassium replacement for dehydration, hypokalaemia or ileus.

  • Avoid loperamide in CDI

    Anti-motility agents can retain toxin and precipitate toxic megacolon. They should be avoided when CDI is suspected or confirmed.

  • Fidaxomicin - first line for CDI

    Oral fidaxomicin 200 mg twice daily for 10 days is preferred where recurrence risk matters, based on the EXTEND trial and NICE NG199.

  • Oral vancomycin

    Oral vancomycin 125 mg four times daily for 10 days is an alternative first-line option for mild to moderate CDI.

  • Severe or fulminant CDI

    Oral vancomycin (higher dose or via NG tube) combined with intravenous metronidazole; urgent surgical review for colectomy in toxic megacolon.

  • Bezlotoxumab and FMT

    For recurrent CDI, bezlotoxumab (a monoclonal antibody against toxin B) and faecal microbiota transplant (NICE TA1071, 2024) reduce further relapses.

  • Infection control and stewardship

    Side room isolation, soap-and-water hand hygiene (alcohol gel does not kill spores), enteric precautions and antibiotic stewardship prevent onward transmission.

What this guide is based on

The sources behind every claim on this page.

UK national guidance and peer-reviewed trials, current at the time of last review.

Key references

Guidelines and trials we relied on.

A quiet reminder

This guide is for information, not medical advice.

Your GP or hospital team knows your history and can tell you which parts apply to you. If diarrhoea is severe, bloody or accompanied by fever, get seen.

  • NICE. Clostridioides difficile infection: antimicrobial prescribing (NG199).

  • UK Health Security Agency. Clostridioides difficile: guidance, data and analysis.

  • NICE. Faecal microbiota transplant for recurrent Clostridioides difficile infection (TA1071, 2024).

  • EXTEND trial (Guery et al, Lancet Infect Dis) supporting extended-pulsed fidaxomicin for recurrence reduction.

Red flags

When AAD needs urgent attention.

Most cases settle in primary care. These are the situations that do not, and where hospital assessment is needed.

  • Fulminant CDI

    Hypotension, shock, ileus, toxic megacolon or lactate above 5 mmol/L needs urgent hospital admission and surgical review.

  • Toxic megacolon

    Colonic diameter above 6 cm with systemic toxicity is a surgical emergency; colectomy may be lifesaving.

  • Rising white cell count

    WCC above 15 x 10 9/L or a rapid rise indicates severe disease and warrants escalation.

  • Acute kidney injury

    Creatinine rising above 1.5 times baseline in the context of CDI defines severe infection.

  • Recurrent CDI

    Around 20 to 25 per cent of first episodes recur, and the risk climbs further after each relapse. Consider fidaxomicin, bezlotoxumab or FMT.

  • Older, frail or immunosuppressed

    Age over 65, care-home residents, chemotherapy patients and transplant recipients are at higher risk of severe disease.

  • Blood in stool with fever

    Suggests colitis and warrants urgent assessment, stool testing and inpatient care.

  • Persistent diarrhoea beyond 8 weeks

    Consider post-infective irritable bowel, microscopic colitis, coeliac disease or inflammatory bowel disease.

  • Household or ward outbreak

    Cluster of cases needs infection-control notification and reinforced enteric precautions.

Living with it

A treatable illness, with a clear plan.

Four things that make the biggest difference day to day: fluids, hygiene, staying away from vulnerable settings and flagging any recurrence early.

A quiet reminder

Small habits stop big outbreaks.

Hand-washing with soap and water, cleaning bathroom surfaces with a chlorine product and staying home for 48 hours symptom-free are the simplest, most effective tools we have.

  1. 01 Hydration

    Fluids first

    Sip oral rehydration solution steadily. Watch for reduced urine output, dizziness or confusion, and get seen if these appear.

  2. 02 Hygiene

    Soap and water beats alcohol gel

    C difficile spores are not killed by alcohol gel. Wash hands with soap and water after using the toilet and before eating.

  3. 03 Isolation

    Stay off work for 48 hours

    Avoid work, school or visiting hospitals until you have been symptom-free for at least 48 hours, and clean surfaces with a chlorine-based product.

  4. 04 Follow-up

    Flag any recurrence

    One in four people relapse. Contact your GP or hospital team early if diarrhoea returns, especially within 8 weeks of treatment.

Frequently asked

Everything we get asked about AAD and C difficile.

Quick answers on testing, treatment choice, recurrence and probiotics.

  • What is antibiotic-associated diarrhoea?

    It is diarrhoea that starts during or up to 8 weeks after taking antibiotics. It ranges from a mild, self-limiting upset caused by changes to gut motility and the microbiome, through to Clostridioides difficile infection (CDI), which can be severe and even life-threatening.

  • Which antibiotics are the biggest culprits?

    Clindamycin, third and fourth generation cephalosporins, fluoroquinolones and co-amoxiclav carry the greatest risk. Tetracyclines and macrolides carry lower risk. Any antibiotic can trigger AAD, and proton pump inhibitors and recent hospital admission add to the risk of CDI.

  • How is C difficile infection diagnosed?

    The UK pathway uses a two-step stool test: a glutamate dehydrogenase (GDH) screen followed by toxin A/B enzyme immunoassay or PCR. A positive GDH with a positive toxin test confirms active CDI; a positive GDH with a negative toxin usually means colonisation rather than infection.

  • Should I take loperamide for antibiotic diarrhoea?

    Not if CDI is suspected or confirmed. Anti-motility drugs can trap toxin in the bowel and precipitate toxic megacolon. In mild non-infectious AAD, loperamide can be used cautiously, but only after CDI has been reasonably excluded.

  • What is the first-line treatment for CDI?

    NICE NG199 recommends oral fidaxomicin 200 mg twice daily for 10 days or oral vancomycin 125 mg four times daily for 10 days as first-line options for mild to moderate CDI. Fidaxomicin is often preferred because it reduces recurrence, based on the EXTEND trial.

  • Do probiotics prevent it?

    The evidence is modest. Certain strains, such as Saccharomyces boulardii and some Lactobacillus preparations, show a small benefit for prevention in specific groups, but they are not routinely recommended for treatment of established AAD or CDI, and are avoided in immunocompromised patients.

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