Dehydration: Symptoms, Causes, Treatment, and Prevention

The symptoms change with how dehydrated you are, from mild to severe. Infants and children may show different symptoms. Illness, vomiting, diarrhoea, or fever, uncontrolled diabetes or kidney issues, medicines that make you pass more urine, or not drinking enough water can all contribute to dehydration.

What Is Dehydration?

Dehydration happens when your body loses more fluid than you take in, leaving you short of water and often salts such as sodium and potassium (electrolytes) which your body needs to work properly.

Your body constantly loses water through urine, sweat, breathing, and bowel movements. You can usually replace these losses by drinking fluids and eating water-rich foods. However, if you don’t drink enough or lose more fluid than usual, you may become dehydrated.

Signs and Symptoms of Dehydration

Spotting dehydration early lets you fix it before it gets worse. The symptoms of dehydration vary depending on how much fluid you have lost, your age, and your overall health. You may first notice thirst or darker urine, while more serious dehydration can cause dizziness, weakness, and passing less urine.

Early Signs

Your body may show several early signs when you need more fluids, including:

• Feeling thirsty more often than usual.
• Having a dry mouth, lips, or tongue.
• Passing urine less frequently.
• Noticing dark yellow urine with a stronger smell.

You may notice these symptoms after exercise, in hot weather, or when you forget to drink enough throughout the day. Drinking water regularly can help you replace fluid losses when dehydration is mild.

However, do not rely on thirst alone. Older adults often don’t feel thirsty even when they need fluids, and young children can’t tell you.

Common Symptoms in Adults

As dehydration progresses, you may experience more noticeable symptoms, such as:

• Dizziness or light-headedness, especially when standing up.
• Persistent tiredness or weakness.
• Headaches or muscle cramps.
• Reduced urine output.

These symptoms can also occur with other health conditions. If they persist despite drinking fluids, or if they worsen, see a doctor rather than assuming dehydration is the only cause.

Signs in Children and Babies

Babies and young children can become dehydrated more quickly than adults, particularly when they have vomiting or diarrhoea. They can’t ask for a drink, so it’s on you to offer fluids. Watch for these signs:

• Fewer wet diapers than usual.
• A dry mouth or tongue.
• Crying with few or no tears.
• Sunken-looking eyes.
• Unusual irritability or drowsiness.
• Poor feeding or difficulty keeping fluids down.

In babies, a sunken soft spot (fontanelle) on top of the head can also indicate dehydration. If your child seems unusually sleepy, passes very little urine, or can’t keep fluids down, get them seen by a doctor without waiting.

Signs of Severe Dehydration

Severe dehydration can slow your blood flow and stop your organs from working properly. You need urgent medical help. Look out for warning signs such as:

• Confusion or unusual behaviour.
• Fainting or difficulty staying awake.
• Very little or no urine.
• Rapid breathing or a fast heartbeat.
• Severe dizziness or weakness.
• Cold, pale, or blotchy skin.

Don’t try to treat these at home. Go to the nearest hospital, get emergency help, especially if someone is confused, collapses, or is hard to wake.

What Causes Dehydration?

Dehydration develops when you lose too much fluid, don’t drink enough to make up for what you lose, or both. Common causes of dehydration include:

Your risk may also be higher if you are an older adult, have certain long-term health conditions or depend on someone else to help you get drinks. Babies and young children face the highest risk of dehydration and heat-related illness because their bodies cannot regulate temperature as efficiently as adults.

If dehydration happens repeatedly without an obvious reason, consult a healthcare professional to identify the underlying cause.

Side Effects and Complications of Dehydration

Your body needs enough water to keep blood volume steady, control your temperature, and keep your kidneys working.

Mild cases may cause temporary discomfort, but severe or prolonged dehydration can lead to serious complications, including an imbalance of salts in your blood, kidney problems, shock, and reduced blood circulation. The risk is especially high for babies, older adults, and people with certain medical conditions.

Can Dehydration Cause Headaches?

Yes, dehydration can cause headaches in some people. A headache after exercising, spending time in the heat, or going several hours without drinking enough fluids may be due to dehydration.

Get help straight away for a sudden, severe headache. See a doctor if headaches keep coming back, or if you also feel confused, faint, have a stiff neck, or experience vision changes.

Does Dehydration Cause Fever?

Dehydration does not usually cause a fever on its own. However, fever can contribute to dehydration because your body may lose more fluid as you sweat more and breathe out more water. A stomach infection may cause both fever and diarrhoea, increasing your risk of fluid loss.

In hot weather, heat exhaustion can raise your temperature, and babies can run a mild fever when badly dehydrated.

Can Dehydration Cause Dizziness and Fatigue?

Yes. When you become dehydrated, you may feel weak, tired, or dizzy. Fluid loss can reduce the amount of blood in your body, which can make you light-headed, especially when you stand up.

How to Treat Dehydration

The right treatment for dehydration depends on how much fluid you have lost and what caused the loss. If you have mild dehydration, drinking fluids may be enough. For severe dehydration, get urgent medical treatment.

How to Cure Dehydration at Home

For mild cases, these steps can help you replace lost fluids:

• Drink water regularly: Sip frequently throughout the day instead of waiting until you feel very thirsty.
• Start with small sips if you feel nauseated: If drinking makes you feel sick, begin with small amounts and gradually increase them as you can keep it down.
• Replace fluids lost through illness: If you have diarrhoea or vomiting, reach for ORS to replace lost water and salts.
• Eat foods with high water content: Foods containing water, such as soups, buttermilk, coconut water, yoghurt, oranges, and watermelon, can contribute to your fluid intake.

These measures may help with mild cases, but they are not a substitute for medical care if your symptoms are severe or getting worse.

What Is the Best Dehydration Solution?

For everyday fluid needs, plain water is fine. If you have lost a lot of water and electrolytes through diarrhoea or vomiting, an oral rehydration solution can help replace both.

ORS is particularly useful when you lose fluids through diarrhoea or vomiting. If you cannot keep the solution down, keep vomiting, or notice signs of severe dehydration, seek medical care. Babies and children may need specific advice about how much ORS to take, so consult a doctor.

ORS contains measured amounts of glucose and electrolytes. These ingredients help your intestines absorb water and salts. Mix one sachet into the exact amount of clean water printed on the pack. Use it within the time the pack gives (usually 24 hours) and throw away what’s left. Check this against your pack and the WHO guidance.

Don’t add extra sugar or salt, and don’t make it stronger than the pack says. A stronger mix can make diarrhoea worse.

For babies, do not stop breastfeeding or giving their usual milk unless a doctor advises otherwise.

Do You Need a Tablet for Dehydration?

Tablets don’t replace fluid. A dissolvable ORS tablet only works when you mix it with the right amount of clean water. Besides ORS, avoid salt tablets and electrolyte supplements unless a doctor recommends them. Taking excessive amounts can disturb your electrolyte balance, push the salt in your blood too high, and make you unwell.

If an illness or medicine is causing repeated fluid loss, a doctor may recommend treatment for the underlying problem. Severe dehydration may require fluids given through a drip into a vein (IV fluids) in a hospital.

How to Prevent Dehydration

You can lower your risk of dehydration by drinking more when you lose more water than usual. Your needs will vary according to your activity level, health, diet, and environment.

How to Stay Hydrated in Summer

Hot weather can increase sweating, making dehydration more likely. To protect yourself during warmer months:

• Carry a water bottle when travelling or spending time outdoors.
• Drink water regularly throughout the day, particularly when you are active.
• Spend time in the shade or a cool environment during the hottest part of the day.
• Wear light, loose, light-coloured clothes.
• Include water-rich foods such as watermelon, cucumber, oranges, and soups in your diet.

How to Prevent Dehydration During Exercise

For shorter, less intense workouts, water is enough. If you exercise for more than about an hour, sweat heavily, or train in hot conditions, you may need to replace electrolytes as well.

When Should You See a Doctor for Dehydration?

Home care works for mild dehydration. See a doctor if you:

• Feel persistently dizzy when standing.
• Pass much less urine than usual.
• Have ongoing vomiting or diarrhoea.
• Feel extremely thirsty or very weak or drowsy.
• Can’t keep fluids down or drink enough.

Seek emergency medical care if you or someone you are caring for becomes confused, faints, is hard to wake, or develops signs of cold, clammy skin and a fast pulse.

Mool Health’s Perspective on Dehydration

At Mool Health, we encourage you to pay attention to everyday changes in your body rather than waiting for symptoms to become uncomfortable.

Before fluid loss becomes more serious, you can build a habit of drinking water regularly and watch for early signs of dehydration.

If you keep losing fluids to loose motions, vomiting, or an unsettled stomach, treat it as a signal worth chasing. The free Mool Health Gut Test asks about your habits and symptoms to find what’s behind it, and Mool doctors build a personalised plan from there. Results vary, and for severe symptoms, see a doctor first.

References

1. Maughan RJ, Shirreffs SM. Dehydration and rehydration in competitive sport. Scand J Med Sci Sports. 2010;20 Suppl 3:40-47. doi:10.1111/j.1600-0838.2010.01207.x
2. Latzka WA, Montain SJ. Water and electrolyte requirements for exercise. Clin Sports Med. 1999;18(3):513-524. doi:10.1016/s0278-5919(05)70165-4
3. Aghsaeifard Z, Heidari G, Alizadeh R. Understanding the use of oral rehydration therapy: A narrative review from clinical practice to main recommendations. Health Sci Rep. 2022 Sep 11;5(5):e827. doi: 10.1002/hsr2.827. PMID: 36110343; PMCID: PMC9464461.
4. Scotland’s national health information service. Dehydration | NHS inform (Last Updated: 01 October 2026)
5. National Health Service (NHS) United Kingdom. Dehydration - NHS (Last Reviewed: 1 May 2026)
6. A.D.A.M. Medical Encyclopedia [Internet]. Johns Creek (GA): Ebix, Inc., A.D.A.M. Dehydration; [updated 2025 Jul 1; cited 2026 Oct 13]. Available from: https://medlineplus.gov/ency/article/000982.htm
7. MedlinePlus [Internet]. Bethesda (MD): National Library of Medicine (US). Dehydration; [cited 2026 Oct 13]. Available from: https://medlineplus.gov/dehydration.html
8. Wang B, Wei X, Zhao X, Wang W, Deng J, Yang H. A Review on In Vivo Research Dehydration Models and Application of Rehydration Strategies. Nutrients. 2024 Oct 21;16(20):3566. doi: 10.3390/nu16203566. PMID: 39458559; PMCID: PMC11510460.
9. WHO Breastfeeding
10. Popkin BM, D'Anci KE, Rosenberg IH. Water, hydration, and health. Nutr Rev. 2010 Aug;68(8):439-58. doi: 10.1111/j.1758-4887.2010.00304.x. PMID: 20646222; PMCID: PMC2908954.