When you are sick, hydration stops being a simple question of whether you reached your usual daily target.
Fever, vomiting and diarrhoea can all contribute to dehydration. Vomiting can also make drinking difficult, while diarrhoea creates continuing losses of fluid and electrolytes. MedlinePlus lists fever, vomiting and diarrhoea among common causes of dehydration, while the NHS advises replacing fluids and, when vomiting or diarrhoea cause larger losses, replacing sugar, salts and minerals as well.
The useful question is not simply, “How much water should I drink?”
It is:
What am I losing, what can I keep down, and do I need more than plain water?
This guide focuses mainly on adults. Babies, young children, older adults and people with some medical conditions can become dehydrated more easily or may need different advice. MedlinePlus and NIDDK identify these groups as needing extra care.
The short version
| What is happening | A practical next step |
|---|---|
| Fever, but you can drink normally | Keep fluids available and drink regularly. Fever can contribute to fluid loss. MedlinePlus |
| Nausea or vomiting | Start with small sips rather than a large drink, then gradually drink more if you can keep it down. NHS |
| Diarrhoea | Replace both fluid and electrolytes. Oral rehydration solution can be useful, especially if diarrhoea is severe or dehydration is developing. NIDDK |
| You cannot keep fluids down | Seek medical advice rather than repeatedly forcing large drinks. NHS |
| Confusion, severe weakness, very little urine, fainting, blood in vomit or stool, or severe abdominal pain | These can be warning signs that need urgent medical assessment. NHS NIDDK |
That table is the core of the article. The details below explain why the advice changes from one situation to another.
If you have a fever
A fever can contribute to dehydration, especially when illness also reduces how much you feel able to drink. MedlinePlus lists fever as a cause of fluid loss and notes that nausea, sore throat and feeling unwell can also make it harder to take in enough fluid.
If you are able to drink comfortably, the practical goal is simple: keep fluids accessible and drink regularly. Watch for signs that you may be falling behind, such as strong thirst, dry mouth, dark urine, urinating less than usual, tiredness or dizziness. These are among the dehydration symptoms listed by MedlinePlus and the NHS.
A fever by itself does not automatically mean you need a special electrolyte drink. Oral rehydration solutions are most clearly recommended when fluid and electrolyte losses become important, such as with vomiting, diarrhoea or signs of dehydration. NIDDK and the NHS both describe ORS in that context.
If you are vomiting, think small first
When you feel sick, a full glass of water can be difficult to keep down.
The NHS advises starting with small sips if you feel sick or have been sick, then gradually drinking more. NIDDK gives similar advice for viral gastroenteritis: if vomiting is a problem, sip small amounts of clear liquids.
This turns the vague instruction to “drink plenty” into something more practical. Official guidance starts with small sips when nausea or vomiting makes drinking difficult, rather than asking you to finish a large glass at once. NHS NIDDK
There is no universal number of millilitres or minutes that every adult should follow at home. The official guidance above focuses on tolerability: start small and increase as you are able.
If you keep vomiting and cannot keep fluid down, the NHS recommends urgent medical advice. At that point the problem is no longer choosing the perfect drink.
Diarrhoea is not only a water-loss problem
Diarrhoea causes the body to lose both fluid and electrolytes. NIDDK recommends replacing both, and the World Health Organization describes oral rehydration solution as a way to replace water and electrolytes lost in stool.
For mild acute diarrhoea, many adults can stay hydrated with ordinary fluids. NIDDK lists water, broths and other electrolyte-containing drinks among options. If diarrhoea is severe, dehydration is developing, or the person is in a higher-risk group, NIDDK specifically recommends oral rehydration solution. NIDDK
The important distinction is that a day with diarrhoea is not the same as an ordinary day when you simply forgot to drink enough water.
What oral rehydration solution actually is
Oral rehydration solution, usually shortened to ORS, is not just water with a health label on it.
The WHO describes ORS as a glucose-electrolyte solution developed to prevent and treat dehydration from diarrhoea. Its public health guidance explains that the solution is absorbed in the small intestine and replaces water and electrolytes lost in stool. WHO: Diarrhoeal disease
The glucose is part of the formulation for a reason. WHO technical guidance on oral rehydration explains that glucose-mediated sodium absorption remains largely intact during diarrhoea, which allows sodium and water to be absorbed through the gut. WHO oral rehydration guidance
For a person at home, the practical lesson is much simpler: ORS is designed to replace water and electrolytes together in proportions intended for rehydration.
Mix ORS exactly as directed
If you buy ORS powder or tablets, follow the instructions for that product.
WHO guidance for ORS packaging specifies that the contents of a packet are dissolved in a defined amount of drinking water and that instructions should clearly show the required water volume. Different products can be packaged for different volumes, so the packet in your hand matters more than a recipe remembered from the internet. WHO
Do not make the solution stronger because you think more salts will work faster, and do not dilute it randomly to improve the taste. The purpose of ORS is its measured composition. Follow the label or ask a pharmacist how to prepare the product you bought. The NHS specifically recommends asking a pharmacist which oral rehydration solution is suitable for you or your child.
ORS and sports drinks are not the same thing
This is an area where advice can sound contradictory until you look at the situation being discussed.
For otherwise healthy adults with a mild stomach bug, NIDDK lists water, fruit juice, sports drinks and broths among fluids that can help replace losses.
But for severe diarrhoea, the CDC recommends oral rehydration solution and states that sports drinks do not replace diarrhoeal losses correctly and should not be used as the treatment for diarrhoeal illness.
So a sports drink and ORS should not be treated as interchangeable products. A sports drink may contribute fluid in a mild illness, but ORS is the product specifically formulated for oral rehydration when diarrhoea is causing important fluid and electrolyte losses.
What about juice, fizzy drinks, coffee and alcohol?
During active diarrhoea, the NHS advises against fruit juice and fizzy drinks because they can make diarrhoea worse.
During active dehydration, NHS guidance advises avoiding caffeinated drinks and alcohol while you are trying to rehydrate. NHS
For everyday hydration outside illness, our guide What Counts Toward Your Daily Water Intake? looks separately at coffee, tea, milk, sparkling water and food.
How can you tell if dehydration may be developing?
Common adult symptoms of dehydration include:
- strong thirst;
- dry mouth;
- urinating less than usual;
- dark-coloured urine;
- tiredness;
- dizziness.
These symptoms are listed by MedlinePlus. The NHS also highlights reduced urination, dark urine, dizziness and unusual tiredness.
No single sign tells you exactly how dehydrated you are. What matters is the pattern, especially when symptoms are appearing alongside ongoing vomiting, diarrhoea or an inability to drink enough.
Severe dehydration can be life-threatening and may require intravenous fluids in hospital. MedlinePlus describes severe dehydration as an emergency, and NIDDK notes that severe dehydration may need IV fluids and hospital treatment.
When home hydration is no longer enough
Seek medical advice promptly if you cannot keep fluids down, continue to show signs of dehydration despite using oral rehydration solution, or have bloody diarrhoea. These are situations the NHS specifically lists as reasons for urgent assessment.
NIDDK advises adults to seek medical care for symptoms including frequent vomiting, high fever, severe abdominal or rectal pain, black or bloody stools, symptoms of dehydration, or inability to drink enough fluid or ORS to prevent dehydration.
Emergency symptoms include confusion or not responding normally, sudden severe abdominal pain, vomiting blood, or green vomit in an adult. The NHS advises emergency assessment for these signs.
The exact medical service to contact depends on the country where you are. The important point is to stop treating the problem as a routine hydration question when you cannot replace what you are losing or serious warning signs appear.
Children and babies need their own guidance
Do not use an adult hydration routine as a substitute for paediatric advice.
Infants and young children are at higher risk of dehydration, particularly when they have vomiting or diarrhoea. MedlinePlus identifies infants and young children as a higher-risk group, and NIDDK notes that diarrhoea can lead to severe dehydration quickly in newborns and infants.
The NHS advises continuing breast or bottle feeding and using small feeds more often if a baby is vomiting. It also recommends urgent advice for a baby under 12 months when there is concern, and for young children with signs of dehydration.
If you are caring for a child with vomiting or diarrhoea, follow age-specific medical guidance and ask a doctor or pharmacist about the appropriate oral rehydration product. NIDDK recommends ORS for children with viral gastroenteritis and advises speaking with a doctor about giving it to an infant.
Older adults and some medical conditions also need extra care
Older adults are more likely to become dehydrated, in part because some people lose some of their sense of thirst with age. MedlinePlus
NIDDK advises older adults, people with weakened immune systems, and people with conditions such as diabetes or kidney disease to talk with a doctor before using oral rehydration solutions for diarrhoea. NIDDK
If a clinician has already given you a specific fluid or rehydration plan because of a medical condition, use that plan rather than a general article or a water-tracking target.
How to use a hydration tracker while you are sick
A tracker can still be useful on a sick day, but its job changes.
Instead of treating your normal target as a medical prescription, use the log to remember what you have actually managed to drink and when. Fever, vomiting and diarrhoea can change fluid losses, and neither a daily goal nor a reminder can measure how much fluid you have lost through illness. MedlinePlus
If you are following ORS instructions from a pharmacist or clinician, those instructions take priority over an app target. If you are unable to keep fluids down or signs of dehydration are getting worse, the next step is medical advice, not another notification.
For ordinary days, How Often Should You Drink Water? explains how to build a practical drinking rhythm. Sick days are different because the question is no longer just about habit.
A simple sick-day hydration plan
- Keep fluids within reach. Fever, vomiting and diarrhoea can contribute to dehydration. MedlinePlus
- If you feel sick, start with small sips. Increase gradually if you can keep them down. NHS
- If diarrhoea is causing significant losses, think about electrolytes as well as water. ORS is designed for this purpose. WHO
- Prepare ORS exactly as directed on the product. Ask a pharmacist if you are unsure which product or preparation is appropriate. WHO NHS
- Do not assume a sports drink is equivalent to ORS. CDC recommends ORS for severe diarrhoeal losses and says sports drinks should not be used as treatment for diarrhoeal illness. CDC
- Watch for dehydration. Less urine, dark urine, dry mouth, tiredness and dizziness are useful warning signs. MedlinePlus
- Get medical help if you cannot keep fluids down or serious warning signs appear. NHS NIDDK
The most useful rule is not “drink as much as possible.”
It is:
Replace what you can, in a form you can keep down, and recognise when the problem has moved beyond home hydration.
Sources and further reading
This article provides general information and is not a substitute for medical diagnosis or individual treatment advice. If you are pregnant, caring for a baby or young child, have a chronic medical condition, take medicines that affect fluid balance, or have been advised to limit fluids, use guidance from your healthcare professional.
