Acute diarrhoea
Causes, first steps, testing thresholds, and when antibiotics help
What is acute diarrhoea?
Acute diarrhoea is diarrhoea lasting up to 14 days. The majority of cases are self-limited and resolve with supportive care.
The main risks are dehydration, electrolyte disturbance, and missing a small number of serious infections.
Common causes
1. Viral gastroenteritis
Typical features: watery diarrhoea, nausea/vomiting, cramping, multiple contacts unwell. Antibiotics do not help.
2. Toxin-mediated food poisoning
Typical features: very rapid onset (hours), prominent vomiting, short duration (<48h). Fluids and rest only.
3. Bacterial gastroenteritis
Common organisms: Campylobacter, Salmonella, Shigella, enterotoxigenic E. coli. Fever and/or blood may occur.
4. Parasitic infection
More likely when symptoms persist >7–10 days, after travel, or untreated water exposure. Examples: Giardia, Entamoeba, Cryptosporidium.
5. Medication-related diarrhoea
Common culprits include antibiotics, metformin, magnesium supplements, PPIs, SSRIs, chemotherapy/immunotherapy, GLP‑1 receptor agonists.
First priorities in all patients
1. Rehydration comes first
Early use of Oral Rehydration Solution (ORS) prevents most complications. Start ORS immediately and continue even if diarrhoea persists.
2. Symptom control (when safe)
- Loperamide: use only if no fever and no blood.
- Avoid if dysentery or suspected C. diff.
- Antiemetics can allow successful ORS intake.
When to seek medical review
Urgent review is required if blood/black stool, high fever, severe pain, dehydration, persistent vomiting, or high-risk status.
When to test stool
Testing is appropriate when severe, fever/blood, not improving by ~72 hours, persists >7 days, high-risk, or recent travel/antibiotics.
Modern practice favours multiplex PCR stool testing.
Antibiotics in acute diarrhoea — key rules
Antibiotics are the exception, not the rule.
Antibiotics may be considered when:
- Severe traveller’s diarrhoea
- Dysentery with fever
- Suspected cholera with severe dehydration
- High-risk hosts with significant illness
Antibiotics should be avoided when:
- Illness is likely viral
- Symptoms are mild and improving
- Shiga-toxin producing E. coli is suspected, due to the risk of haemolytic uraemic syndrome (HUS)
Common mistakes that cause harm
- Starting antibiotics “just in case”
- Using loperamide in febrile or bloody diarrhoea
- Ignoring dehydration
- Delaying assessment in high-risk patients
What happens if symptoms do not settle?
Persistence beyond 14 days suggests parasites, early IBD, or post-infectious gut dysfunction. See Chronic diarrhoea and Timeline.
Key takeaway: Most acute diarrhoea resolves with correct hydration and supportive care. Antibiotics are rarely needed and can cause harm when used incorrectly.