Child with vomiting and diarrhoea: what actually treats dehydration

For most children with vomiting and diarrhoea from gastroenteritis, the treatment that works is oral rehydration — giving fluid by mouth, in small frequent amounts, ideally an oral rehydration solution — rather than a drip in hospital or, at the other extreme, withholding fluids [s1][s2]. Gastroenteritis is usually a self-limiting viral illness, and the real danger is dehydration, so the whole of management is built around keeping fluid going in and spotting the child who is becoming dehydrated [s1].

The condition, and why fluid is the point

Acute gastroenteritis causes the sudden onset of diarrhoea, often with vomiting, and in the UK it is usually viral [s1]. Most children can be looked after at home, and the NICE guideline on diarrhoea and vomiting in under-fives is explicit that the clinical priority is assessing and managing hydration, not the diarrhoea itself, which will settle [s1]. NICE advises continuing breastfeeding and other milk feeds, encouraging fluids, and — importantly — discouraging fruit juices and fizzy drinks, especially in children at increased risk of dehydration [s1]. Where a child is dehydrated or at risk, an oral rehydration solution given in small, frequent sips is the recommended treatment [s1].

Oral fluids versus a drip

The instinct that a seriously vomiting child needs an intravenous drip is usually wrong. A Cochrane review compared oral rehydration therapy with intravenous therapy across 17 trials involving 1811 children [s2]. It found only a small difference in treatment failure — 4% more with oral therapy (risk difference 4%, 95% confidence interval 1 to 7), meaning that for roughly every 25 children treated orally, one would fail and need a drip [s2]. Against that, oral rehydration shortened hospital stays, by a weighted mean of 1.20 fewer days (95% CI 2.38 to 0.02 days shorter), and avoided the risks of a drip: phlebitis (vein inflammation) occurred more often with intravenous therapy [s2]. The reviewers found no meaningful advantage to intravenous fluids for children who can drink, which is why oral rehydration is the first-line treatment in guidelines [s1][s2].

Does anti-sickness medicine help?

Persistent vomiting is often what tips families towards hospital, because a child who keeps vomiting cannot keep oral fluids down. Here the evidence is more favourable to one drug than reputation once allowed. A Cochrane review of antiemetics in acute gastroenteritis pooled seven trials in 1020 children and, for oral ondansetron compared with placebo, found a reduction in immediate hospital admission (risk ratio 0.40, number needed to treat 17), a reduction in the need for intravenous rehydration during the emergency-department stay (RR 0.41, NNT 5), and an increase in the proportion of children whose vomiting stopped (RR 1.34, NNT 5) [s3]. The main trade-off was more diarrhoea as a side effect, reported in most of the ondansetron studies [s3]. This is a medicine used under clinical judgement, not something to source or dose at home [s3].

Red flags — when to get help now

Dehydration is the emergency to watch for. Seek urgent medical advice if a child has signs of dehydration — such as drowsiness or unusual irritability, sunken eyes, a dry mouth and tongue, or passing much less urine than usual — or if they are unable to keep any fluids down [s1]. Other warning features that warrant prompt assessment include blood in the diarrhoea or vomit, green (bile-stained) vomit, a swollen or very painful abdomen, a high fever, or symptoms that are getting worse rather than better [s1]. Very young infants dehydrate fastest and should be assessed with a lower threshold [s1].

For related everyday questions, our review of childhood constipation and what works covers the opposite bowel problem, and toddlers who eat little during illness are discussed in toddler iron deficiency. This article is informational and not medical advice; a clinician can assess an individual child, judge the degree of dehydration and decide on treatment [s1].