Dehydration and Oral Rehydration: What Vomiting and Diarrhea Take Out

By the second day of a stomach bug the question is not what to eat but what to drink, and "lots of water" is the wrong answer more often than people expect. Diarrhea and vomiting take out water and salts together, and plain water replaces only one of them, while sweet drinks pull more fluid into the gut. What puts both back is oral rehydration solution: a measured mix of salts and glucose that the intestine absorbs even while it is losing fluid. Most cases are managed at home, but the signs that say otherwise are specific, and they come first.

When to see a doctor

Right away (999 in the UK, 112 in the EU and Russia, 911 in the US): vomiting blood; green or yellow-green vomit in a child; a severe headache or a stiff neck; severe abdominal pain; trouble breathing; and the signs of shock from fluid loss — blue, grey, pale or blotchy skin, lips or tongue, cold skin, confusion, extreme drowsiness (NHS, UK). Also seizures, a person who cannot be woken, and any fever in a baby under twelve weeks (CDC, US; NHS, UK).

The same day (111 or a doctor, UK): a baby under twelve months; a child who has stopped feeding; signs of dehydration that persist despite rehydration solution; fluids that will not stay down; blood in the stool; diarrhea beyond seven days or vomiting beyond two; unusual tiredness, dizziness on standing, dark urine, fast breathing, a sunken soft spot in a baby (NHS, UK). The WHO's home list for a child: many stools, great thirst, sunken eyes, no improvement after three days, fever, not eating or drinking.

Routinely: if you have kidney or heart disease, diabetes, or take diuretics, blood pressure medicines or metformin, ask beforehand what to do with them on sick days (NHS Scotland) — and how much you may drink, if you have a fluid limit.

What is happening

The signs. In adults and children: thirst, headache, light-headedness, dark strong-smelling urine, urinating less than usual, tiredness, a dry mouth and lips, sunken eyes. In babies: a sunken soft spot, few or no tears, fewer wet diapers, drowsiness or irritability (NHS, UK). Diarrhea usually stops within five to seven days and vomiting within one or two (NHS); the WHO counts three or more loose stools a day as diarrhea.

Why the solution is measured. Since 2004 the WHO and UNICEF recommend a reduced-osmolarity solution: 75 mmol/l of sodium, 75 of glucose, 20 of potassium, 245 mOsm/l in total. Compared with the older, saltier formula it cut stool volume, vomiting and the need for drips. The WHO's 2024 guideline extends the recommendation to children up to ten.

Not every sachet is that solution. In Russia, Rehydron — the pharmacy default — is 282 mOsm/l with about 90 mmol/l of sodium, and its own leaflet says that children need solutions with less sodium and lower osmolarity. Read the label for a children's, low-osmolarity product. Rehydron's leaflet also warns that with kidney or heart failure or diabetes the treatment belongs under a doctor's supervision.

What to do

How much (WHO, at home). Under two years: 50–100 ml after each loose stool and between them; from two: 100–200 ml; older children and adults, as much as they want — unless a doctor has set a fluid limit. Small frequent sips; if the child vomits, wait ten minutes and go on more slowly; continue until the diarrhea stops.
What counts as fluid. For an exclusively breastfed baby: breast milk plus rehydration solution, nothing else (WHO). For everyone else, rehydration solution first; otherwise water, unsweetened tea, salted soup or rice water (WHO). Not soft drinks, sweetened tea or juice, coffee, or herbal "remedies" (WHO; NHS: no fruit juice or fizzy drinks — they make diarrhea worse). Sports drinks are acceptable for a mild case in an adult (CDC), not for a child.
Mix it exactly. One sachet in the volume printed on it, nothing added, kept in the fridge and used within 24 hours (Rehydron leaflet). A stronger solution is not a better one.
Keep feeding. Breastfeed more often and longer; do not dilute formula; offer food as soon as the child wants it, avoiding fatty and spicy dishes (NHS; WHO). The solution is in addition to normal drinking, not instead of it.
Skip the home recipe. Sugar-and-salt recipes exist for places without pharmacies, but a mistake in the salt is dangerous and a kitchen spoon is not a measure; European pediatric guidelines do not recommend home mixes where sachets are available. In the countries this page covers, a pharmacy has them.
Paracetamol for discomfort is fine (NHS); ibuprofen is not for a dehydrated child, and aspirin is not for children at all — under 16 in the UK (NHS).

When this is not the case

Anti-diarrheal tablets are not rehydration. Loperamide is for adults and, in the UK, for children from 12 without a prescription; in Russia the capsules are licensed from age six; the WHO warns it can be harmful, especially under five. Do not take it with blood or mucus in the stool, with a fever, with severe diarrhea after antibiotics, with constipation or a swollen abdomen, during a flare of ulcerative colitis, or beyond 48 hours without a doctor; with liver disease, in pregnancy or while breastfeeding, ask first (NHS, CDC). The FDA (US) warns that doses above the label cause serious heart rhythm problems.

Antibiotics are not routine. Most gastroenteritis is viral; the WHO reserves antibiotics for bloody diarrhea and cholera. Zinc is a WHO recommendation for children in high-burden regions and is not routine in European guidelines; if it is prescribed, the dose comes from the doctor. Related: paracetamol or ibuprofen, measuring liquids, what helps with a cold, and the map of the cabinet, the active ingredient.

Sources

Checked: September 7, 2026

Dehydration and Oral Rehydration: What Vomiting and Diarrhea Take Out, and What Puts It Back