Quick answer: Prioritise hydration — diarrhoea causes fluid and electrolyte loss. Continue breastfeeding. Offer oral rehydration solution (Dioralyte) in small frequent sips alongside milk. Do not give anti-diarrhoeal medications to babies under any circumstances.
Common causes
Viral gastroenteritis is the most common cause — rotavirus and norovirus are the most frequent culprits, particularly in winter. Other causes: antibiotic use (disrupts gut flora, causes loose stools in approximately 30% of courses); dietary change (new food introduction, formula switch); cow’s milk protein allergy (typically also involves blood or mucus in stools); and normal variation in breastfed babies (loose, frequent stools can be normal in exclusively breastfed babies — compare against your specific baby’s baseline rather than against formula-fed baby patterns).
The priority: hydration
Young babies are more vulnerable to dehydration than adults — their surface-area-to-volume ratio is higher and baseline fluid requirements are greater. Breast milk or formula are the primary rehydration sources and should be offered more frequently than usual during a bout of diarrhoea. Do not give plain water as the primary rehydration fluid in babies under 12 months — free water in large amounts can produce dangerous hyponatraemia (low blood sodium) in young infants. Oral rehydration solution (Dioralyte, available over the counter in the UK) provides electrolytes and glucose in the correct ratio and can be given in small, frequent sips alongside continued breast milk or formula. Signs of adequate hydration: pale yellow urine, at least 4–5 wet diapers per 24 hours.
Red flags: when to seek medical help
Contact your doctor same day or go to the ER for: any baby under 3 months with diarrhoea; a baby of any age with signs of dehydration (fewer than 4 wet diapers in 24 hours, dark concentrated urine, sunken eyes, sunken fontanelle, dry mouth, unusual lethargy, rapid breathing); blood or mucus in the stool; diarrhoea accompanied by bile-coloured (bright green) vomiting; and a baby who is worsening rather than stable or improving at 24–48 hours. Dehydration can develop rapidly in young babies — don’t wait to see if they improve if these signs are present.
What not to do
Anti-diarrhoeal medications (loperamide/Imodium, kaolin): not licensed for children under 12 years for acute gastroenteritis and can cause serious complications including intestinal ileus. Do not use. Diluted formula: diluting formula with extra water disrupts the electrolyte balance. Do not do this. Fruit juice or squash: the sugar load worsens osmotic diarrhoea. Stopping breastfeeding: never stop — breast milk contains antibodies against gut pathogens and is the best possible continued fluid.
Frequently Asked Questions
How long does baby diarrhoea usually last?
Viral gastroenteritis: 5–7 days for the acute phase; loose stools can persist for up to 2 weeks after as the gut flora re-establishes. Diarrhoea lasting more than 2 weeks is chronic diarrhoea and warrants doctor assessment.
Should I change formula during a bout of diarrhoea?
Not routinely — switching formula mid-illness adds a dietary change variable. Continue with the usual formula. If diarrhoea is significantly worsening with formula feeds, discuss with your doctor before switching — in some cases, temporary use of a lactose-free formula is appropriate if post-infectious lactose intolerance is suspected.
My breastfed baby has frequent loose stools normally — how do I know it’s diarrhoea?
Compare against your baby’s baseline. Normal breastfed baby stools are loose, frequent, and yellow-seedy. Diarrhoea is defined by a change from the baby’s personal normal — more frequent, more watery than usual, often with an abrupt onset. If you’re uncertain, call your pediatrician or doctor.
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