How to Rehydrate When You're Sick: What Actually Works

Vomiting and diarrhoea take salt out along with water. That's why plain water underperforms, and why the sugar in oral rehydration solution is doing real work rather than making it palatable.

When you've been up all night with a stomach bug, the instinct is to sip water. It's the obvious move and it's better than nothing.

It's also, on its own, not what the evidence supports, because the fluid leaving you isn't just water. It's water and salt, and replacing only one of them leaves you with a dilution problem on top of a volume problem.

The solution to this was worked out decades ago and is one of the genuinely great public-health interventions. It's also widely misunderstood, including by people who reach for a zero-sugar electrolyte sachet thinking it's the same thing.

Before you read on: this guide covers situations that can become medical emergencies. Read the scope and safety note first.

The Short Answer

Bottom line: use an oral rehydration solution rather than plain water when vomiting or diarrhoea is significant. The glucose in it isn't flavouring, it's the mechanism that carries sodium and water across your gut wall, and removing it removes the point.

Three things worth knowing:

• Sodium and water leave together. Replacing water alone dilutes what's left.
• The formula is deliberate and tested. WHO's reduced-osmolarity version cut stool output about 20% and IV therapy need about 30% versus the older formula.
• Zero-sugar electrolyte drinks are a different product for a different job. They're fine for sweating. They don't do this.

Why Plain Water Falls Short

Because the losses are salty, and the replacement isn't.

Diarrhoea and vomiting move substantial volumes of fluid out of your body, and that fluid carries sodium, potassium and chloride with it. Drinking plain water restores volume while further diluting the electrolytes that remain.

There's also an absorption problem. A gut that's inflamed and moving fast isn't absorbing efficiently, so a large volume of plain water can pass through with much of it never crossing into your bloodstream.

The fix exploits a specific piece of physiology: glucose and sodium are absorbed together across the intestinal wall by a shared transporter. Present them together in the right proportions and glucose actively drags sodium across. And water follows the sodium osmotically.

That's the whole idea behind oral rehydration solution. It isn't "water with electrolytes added." It's a formulation engineered around a transport mechanism, which is why the proportions matter rather than being a matter of taste.

What's Actually In Oral Rehydration Solution

Specific concentrations, chosen deliberately, and revised once on the basis of trial evidence.

The WHO reduced-osmolarity formulation:

Note the sodium and glucose figures: 75 and 75. That near-parity is the point: the transporter needs both present in comparable amounts to work efficiently.

The osmolarity figure matters too. This formula replaced an earlier standard at 311 mOsm/L, and the change was evidence-driven. Per WHO's own report on oral rehydration salts, the reduced-osmolarity version "reduced stool output by about 20 percent and decreased the need for supplemental intravenous therapy by about 30 percent compared with the standard solution."

A 30% reduction in people needing an IV, from adjusting a formulation. That's why this is considered one of the more consequential low-cost interventions in medicine.

WHO's own summary of what ORS is stays deliberately simple: a mixture of clean water, salt and sugar. But the ratios behind that description are what make it work.

Why a Zero-Sugar Mix Is the Wrong Tool

This is the practical error most likely to affect readers of this site, because zero-sugar electrolyte products are exactly what a hydration-conscious person already owns.

The reasoning that leads people astray is sensible on its face: I'm losing electrolytes, I have an electrolyte drink, and mine doesn't have unnecessary sugar in it.

But in this context the glucose isn't unnecessary. It's the transport mechanism. Remove it and you have a salt solution without the thing that carries the salt across the gut wall, which is a materially different product from ORS, however similar the label looks.

To be clear about scope, because we recommend zero-sugar formulas elsewhere on this site:

This is the same distinction we drew in the GLP-1 guide, where the symptom tiers split the same way, and it's why our hangover guide points at ORS rather than at a zero-sugar sachet.

Sports drinks are also not ORS. They're generally higher in sugar and lower in sodium than the formula above, built for exercise palatability, not for replacing diarrhoeal losses.

How to Actually Use It

Small amounts, often. Volume delivered over time beats volume delivered at once, especially when vomiting is in play.

• Sip, don't gulp. A few teaspoons or a small mouthful every few minutes is better tolerated than a glass, and a glass that comes straight back up delivers nothing.
• Keep going after vomiting. Wait ten minutes or so, then restart small. It feels futile; a proportion is absorbed each time.
• Cold or iced often helps, and ice lollies made from ORS are a legitimate way to deliver it slowly, particularly for children who'll refuse a cup.
• Buy sachets rather than mixing your own. They're inexpensive, available from any pharmacy, and correctly proportioned. Home mixing is where this goes wrong. Too much salt is actively harmful, and the margin for error is smaller than a kitchen measurement can reliably hit, especially for a child. This is a place where the ready-made version is genuinely safer.
• Reintroduce food when you can face it. Prolonged fasting isn't recommended for ordinary gastroenteritis; simple food as tolerated is generally fine.
• Skip alcohol and go easy on very sugary drinks. High-sugar drinks without matched sodium can worsen diarrhoea by drawing water into the gut.

When to Get Medical Help

Some situations need a clinician rather than a better drink, and knowing them in advance saves you deciding while ill.

WHO's guidance is to consult a health professional for persistent diarrhoea, blood in the stool, or any signs of dehydration. Persistent here means lasting around two weeks or more, but you should not wait that long if other flags are present.

Seek care promptly for:

• Unable to keep any fluid down at all
• No urination, or markedly reduced urination
• Blood in stool or vomit
• Dizziness on standing, confusion, or unusual drowsiness
• High fever alongside the fluid losses
• Severe or worsening abdominal pain

Lower the threshold considerably for infants and young children, older adults, and particularly anyone living alone, where symptoms may go unnoticed. And anyone with kidney disease, heart failure, diabetes or a condition affecting fluid balance.

For infants specifically: they dehydrate faster than adults, and reliable warning signs differ from adult ones. Contact a clinician early rather than working through a checklist. Reported thresholds for things like time since the last wet nappy vary between sources, which is itself a reason to ask rather than to time it.

For the general adult picture of what dehydration looks like, our guide to signs that don't involve thirst covers the broader symptom set.

No product recommendation appears on this page. Our affiliate catalogue's electrolyte products are zero-sugar formulas: genuinely good for sweat replacement, and by the reasoning above, the wrong glucose ratio for this use. Buy a pharmacy ORS sachet instead. That's the honest answer even though nobody pays us for it.

FAQ

Is plain water enough when you have vomiting or diarrhoea? Often not. Those losses take sodium out along with water, so replacing water alone dilutes what remains. Oral rehydration solution pairs sodium with glucose because glucose actively drives sodium absorption across the gut wall, pulling water with it, which is why it outperforms water when losses are significant.

Can you use a zero-sugar electrolyte drink for stomach illness? It's the wrong tool for this specific job. The glucose in oral rehydration solution isn't flavouring. It's the mechanism that moves sodium and water across the gut wall. A zero-sugar mix removes the component doing that work, so it doesn't replicate what ORS does during active vomiting or diarrhoea.

What is actually in oral rehydration solution? The WHO reduced-osmolarity formula contains sodium 75 mmol/L, chloride 65, glucose 75, potassium 20 and citrate 10, giving a total osmolarity of 245 mOsm/L. It replaced an earlier 311 mOsm/L formula, and WHO reports the reduced version cut stool output by about 20% and the need for supplemental intravenous therapy by about 30%.

When should you see a doctor for dehydration from illness? WHO advises consulting a health professional for persistent diarrhoea, blood in the stool, or any signs of dehydration. Add to that being unable to keep any fluid down, markedly reduced urination, dizziness or confusion, and any illness in an infant, a frail older adult, or someone with kidney or heart disease.

Should you make your own rehydration solution at home? Commercial sachets are safer and inexpensive. Home mixing carries a real risk of getting the salt concentration wrong, and too much sodium is actively harmful, especially for young children. If no sachet is available and you can't reach a pharmacy, that's a situation to ask a clinician or pharmacist about rather than improvise from a web page.

Keep Reading

• The Signs of Dehydration That Don't Involve Feeling Thirsty: the broader adult symptom picture.
• Hydration on GLP-1 Medications: the same symptom-tier logic, applied where GI side effects are medication-driven.
• Dehydration Risk When an Older Adult Lives Alone: why illness-related dehydration is riskier without someone else in the house.

A Note on Scope

This article is for informational purposes only and isn't medical advice. Dehydration from illness can become serious quickly, particularly in infants, young children, older adults, and people with kidney, heart or liver disease or diabetes. Seek medical care if you or someone you're caring for cannot keep any fluid down, has blood in their stool or vomit, has stopped urinating, is confused or difficult to rouse, has a high fever, or has diarrhoea lasting more than a couple of days. For infants, contact a clinician early rather than waiting. They dehydrate faster than adults and the usual warning signs are harder to read. This guide does not cover intravenous rehydration, which is a clinical decision.

Sources

• World Health Organization: Oral Rehydration Salts report: the reduced-osmolarity composition in mmol/L, the change from 311 to 245 mOsm/L, and the roughly 20% stool-output and 30% intravenous-therapy reductions.
• World Health Organization: Diarrhoeal disease fact sheet: ORS as a mixture of clean water, salt and sugar, and its role in replacing water and electrolytes.
• World Health Organization: Diarrhoea health topic: guidance to consult a health professional for persistent diarrhoea, blood in the stool, or signs of dehydration.

Not used: a home-mixing recipe using household spoon measures. The proportions circulate widely, but a mis-measured salt concentration is genuinely hazardous (particularly for children) so this guide points to correctly-formulated commercial sachets instead. Also excluded: specific hour-count thresholds for time since last urination or last wet nappy, which differ between sources; the warning signs are described qualitatively and readers are directed to contact a clinician early.