Dehydration explained

What causes dehydration

Dehydration develops when fluid losses outpace what you drink. The most common triggers are illnesses that cause vomiting and diarrhoea, which flush out water and salts quickly, and this is a particular risk in babies and young children. Sweating heavily in hot weather or during hard exercise, and having a high temperature, also increase losses. Some people become dehydrated because they are not drinking enough — for example when unwell and off their food, when very busy, or in older people whose sense of thirst is weaker. Conditions such as diabetes can cause the body to pass large amounts of urine, and drinking a lot of alcohol has a similar effect. Certain medicines, such as water tablets (diuretics), also increase how much fluid you lose. Recognising your own risk situations helps you drink more before problems start.

Signs and symptoms

In adults, early signs of dehydration include feeling thirsty, having a dry mouth, lips, and eyes, passing small amounts of dark yellow, strong-smelling urine, and going to the toilet fewer than usual. You may feel tired, dizzy or lightheaded, or have a headache. Urine colour is a handy guide: pale straw usually means well hydrated, while dark urine suggests you need to drink more. In babies and young children, watch for a sunken soft spot on the head, fewer wet nappies, few or no tears when crying, drowsiness, and fast breathing. Older people may become confused or more unsteady. More severe dehydration can cause a fast heartbeat, sunken eyes, cool blotchy hands and feet, and very little or no urine — these are signs that urgent help is needed rather than home rehydration alone.

How to rehydrate safely

For mild dehydration, the answer is usually to drink more fluids, little and often. Water is ideal, and for most people diluted squash or fruit juice is fine too. If you have been vomiting, take small, frequent sips rather than large gulps, which can bring the sickness back. When dehydration follows diarrhoea or vomiting, oral rehydration salts from a pharmacy help replace lost water and salts and are especially useful for children and older people; a pharmacist can advise on suitable products. Avoid drinks with a lot of sugar, as well as too much alcohol and caffeine, which can worsen fluid loss. Keep drinking steadily until your urine is pale again. Encourage unwell children and older relatives to sip regularly, as they may not feel thirsty even when they need fluids.

Dehydration in babies, children and older people

Some groups become dehydrated faster and more dangerously, so they need extra care. Babies and young children can lose fluid very quickly through vomiting and diarrhoea and can become seriously unwell in a short time, so they should be watched closely and offered frequent drinks or their usual milk. Keep breastfeeding or bottle-feeding as normal and offer extra fluids. Oral rehydration salts are often recommended for children who are becoming dehydrated. Older people are also vulnerable because their sense of thirst fades and some take medicines that increase fluid loss; dehydration can show up as new confusion, dizziness, or falls rather than obvious thirst. If a baby, young child, or frail older person is not keeping fluids down, has signs of more than mild dehydration, or seems drowsy or unwell, seek medical advice promptly.

Preventing dehydration and when to get help

Prevention is mostly about drinking enough, especially when the risk is higher. Drink regularly through the day, and drink more in hot weather, during exercise, and when you are unwell with a fever, vomiting, or diarrhoea. Keep a drink to hand and do not wait until you feel very thirsty, as thirst is a late signal. Contact a GP, pharmacist, or NHS 111 if you or someone you care for cannot keep fluids down, has diarrhoea or vomiting that is not settling, feels unusually tired or confused, or shows signs of more than mild dehydration. Seek urgent help — including 999 if needed — for severe symptoms such as drowsiness that is hard to rouse, a weak or very fast pulse, fits, or not passing urine. Acting early usually prevents dehydration from becoming serious.