At a glance

  • People become dehydrated if they don't drink enough water or their body loses too much fluid.
  • The first signs are thirst, trouble concentrating, a headache and dizziness.
  • It then helps to quickly drink enough.
  • Severe dehydration can cause symptoms such as confusion, seizures, kidney failure and serious blood circulation problems (shock). Urgent medical help is then needed.


Photo of a woman after doing sports in the sun

Having a headache, trouble concentrating, and feeling dizzy could simply be a sign that you haven’t had enough to drink. Especially on hot days or after doing sports, your body may not have enough fluids. Vomiting, diarrhea and blood loss can also cause your body to lose fluid.

We can't live without water: Our bodies need it for almost all functions, such as regulating our breathing, body temperature, blood pressure and pulse, and for digesting food and removing toxins from the kidneys. About 50 to 60% of adults’ body weight is made up of water. In babies, it's up to 75%.

Slight dehydration can usually be sorted out quite easily by drinking more. If your body doesn't have enough water, that can often cause problems with your sodium levels, too. It is then a good idea to get some electrolytes into your body as well – for example, by drinking a salty broth or an isotonic drink. Severely dehydrated people need medical help.


Mild dehydration often goes unnoticed. If there are noticeable signs, along with thirst they may include

  • a dry mouth and furry feeling on the tongue,
  • trouble concentrating,
  • a headache,
  • exhaustion,
  • quickly getting tired,
  • reduced physical and mental fitness,
  • needing to pee less, or
  • less and darker urine – although the darker color isn't always a reliable sign in older people.

Moderate or severe dehydration can also cause

  • dry mucous membranes,
  • dizziness after standing up quickly,
  • a racing heart (tachycardia),
  • pain in the belly and chest,
  • sunken eyes and no tears when crying,
  • muscle cramps,
  • seizures,
  • lethargy or
  • confusion.

Severe dehydration can lead to “hypovolemic shock.” This is where the lack of fluids is so severe that not enough blood can be pumped around the body. It is a medical emergency. Other signs of hypovolemic shock include

  • cold hands and feet,
  • restlessness, shivering or the sweats,
  • anxiety or apathy, and sometimes even unconsciousness.

If children are dehydrated, it can affect their behavior: They are then exhausted, restless, in a bad mood or weepy, but cry without tears. They sometimes breathe faster or more deeply. Symptoms in babies also include a sunken soft spot on the top of their head, and a fever. Babies and young children's diapers are noticeably less full.

Causes and risk factors

The water and sodium levels in your body are regulated by hormones. Slightly high or low levels in cells, blood vessels or body tissue are constantly balanced out in this way. If the overall fluid levels go down, we start to feel thirsty and our kidneys keep water in our body. In healthy adults, that generally stops the lack of fluids from getting worse.

But sometimes our bodies don’t get enough fluids from our food and drinks. Reasons for this include:

  • Stress or distraction: You forget to drink even though you're thirsty.
  • Less thirst: Especially older people often feel less thirsty.
  • Deliberately reducing the amount you drink: Some people drink less on purpose, perhaps because they're worried about having to go to the toilet too often or want to avoid "leaking" ().
  • Mental impairments: People with dementia, for example, sometimes forget to eat or drink, have less of an appetite, or refuse to have meals and drinks.
  • Physical obstacles: Some people can't eat and drink properly on their own, and might not get enough help.
  • Difficulty swallowing: People may avoid eating or drinking because it feels unpleasant or is even painful.
  • Unclear signals: Babies and toddlers interpret being thirsty as feeling “unwell” and often aren't yet able to say that they're thirsty.

Dehydration can also be caused by losing too much fluid and sodium or losing them too quickly. Possible causes include:

  • Heat or intense physical exertion that makes you sweat a lot and lose a lot of evaporated fluid through your breath.
  • Larger skin surface area in relation to body volume: Compared to adults, children lose more fluid through their skin when they have a fever or when it's hot.
  • Diarrhea or vomiting
  • Infections that lead to fever and a lot of sweating
  • Endocrine (hormone) or kidney diseases
  • High blood sugar (hyperglycemia)
  • Large, severe burns
  • Diuretics (“water pills”)
  • High blood loss – for example, during surgery or after a severe injury

Nursing mothers need more fluids. Some people say that pregnant women need more fluids too. But there is no research on whether pregnant women need to drink more. If pregnant women experience severe nausea with vomiting, they could become dehydrated.


Mild dehydration is common. Especially in older people, it's often caused by a combination of different things, such as illnesses, taking certain medications, feeling less thirsty, and deliberately drinking less. Children often have stomach bugs that may cause severe dehydration. Babies and toddlers are particularly at risk of dehydration.

Climate change is making heat waves more common. Heat waves increase the risk of dehydration and other heat-related health problems.


Even mild dehydration can cause mood swings, reduce your attention span and affect your ability to concentrate and make judgments. If that happens in people who are already mentally impaired, dehydration can make the impairment worse.

If you lose a lot of fluid, it can lead to hypovolemia. Then there is less blood in your body.

Significant dehydration can sometimes have serious effects, especially in older people. These include:

  • Falls and bone fractures
  • Increased likelihood of not tolerating medication
  • Digestion problems like constipation
  • Infections, such as urinary tract infections (UTIs)
  • Confusion
  • Seizures
  • Heart rhythm problems
  • Low blood pressure
  • Very poor blood circulation in heat
  • Impaired kidney function
  • Pressure sores ()
  • Poor wound healing
  • Longer recovery time after surgery
  • Reduced quality of life
  • Heart disease
  • Kidney stones


It is important to seek urgent advice from a doctor if there are symptoms like unusual weakness, confusion or dizziness – especially in children and older people. The same is true if you lose a lot of fluids over several days and struggle to balance it out – for instance, if you vomit or sweat excessively.

The doctor will first ask about how frequently you pee, about any dizziness or concentration problems, and any underlying medical conditions such as diabetes, heart disease or kidney disease.

They will then examine your mucous membranes to see if they're too dry. Another sign of dehydration is if the pink color doesn’t return within about 2 or 3 seconds after putting pressure on the skin, such as on the nail bed. Doctors sometimes also do a pinch test, for instance beneath the collar bone or on the thigh. This involves gently pinching the skin between their thumb and index finger before letting it go and seeing whether it takes longer than usual to flatten out again.

They will also usually measure your blood pressure, listen to your chest, feel your abdomen, and examine your legs for fluid retention. Blood and urine (pee) samples can be sent to a laboratory for testing, too.

To check how severe dehydration is in children, the doctor will ask about their current weight and also weigh the child themselves. The weight difference reflects the degree of dehydration. But the child's exact current weight isn’t always known. Because of this, the doctor will also ask or check whether the child

  • is restless, listless, sleepy or very irritable,
  • has sunken eyes,
  • has fewer or no tears when crying, and
  • pees regularly.


To make sure your body has enough water, you should drink about as much fluid throughout the day as your body loses. Most of that comes from what we drink, but foods (both liquid and solid) also provide fluids. Drinking about 1.5 liters per day is a good rule of thumb for adults in Germany and similar countries.

More fluids are needed if

  • you do very strenuous activities at work or in your free time,
  • it is very hot outside, like during heat waves,
  • you take medications that change the body’s water and sodium levels,
  • you have an – especially one that leads to a fever, sweating, diarrhea or vomiting, or
  • you are a woman who is breastfeeding.


You can balance out mild dehydration yourself by drinking more fluids for a few hours. Isotonic drinks with electrolytes can be a good idea after sweating heavily for an extended period of time, and a salty broth can help if you have a fever or diarrhea.

Children should also drink more if they lose a lot of fluids – for example, due to a high fever, sweating heavily, diarrhea or vomiting. Children can be given an electrolyte, too, preferably taking small sips or spoonfuls regularly over about four hours. They might vomit again then. That's normal. Your family doctor or pharmacy can give you further tips and advice about what babies or toddlers can eat and drink. If the symptoms don’t go away within a few days and the child is still losing a lot of fluids, it's important to see your family doctor.

Severe dehydration can usually no longer be treated just through drinking. The doctor may prescribe an infusion (a drip) with fluids and electrolytes for the child. Then they might have to do a blood test first to see what exactly the child is lacking. If there are signs of severe dehydration, like dizziness or sudden confusion, it's best to go to your family doctor or a hospital. There the missing fluid (and electrolytes or glucose, if needed) can quickly be given through a drip into a vein.

If the cause of dehydration is known, it can be treated – for instance, with medications for diarrhea or nausea. If medication is causing fluid loss, a different medication could be used instead, or the dose adjusted.

Everyday life

It is quite normal to forget to drink enough every now and then. And some people simply don't feel thirsty very often. Older people, in particular, may have a reduced sense of thirst – just like they often have less of an appetite, too. Because of this, people who only rely on feelings of thirst in older age are at risk of not drinking enough.

Various things can help you remember to drink enough in everyday life, such as special apps.

Further information

When people are ill or need medical advice, they usually go to see their family doctor first. Information about health care in Germany can help you to navigate the German health care system and find a suitable doctor. You can use this list of questions to prepare for your appointment.

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Barreto Annes LM, Andrade R, Pontes IEA et al. Subcutaneous Versus Intravenous Rehydration in Hospitalized Older Adults: A Meta-Analysis. J Infus Nurs 2020; 43(5): 283-291.

Bhanu C, Avgerinou C, Kharicha K et al. 'I've never drunk very much water and I still don't, and I see no reason to do so': a qualitative study of the views of community-dwelling older people and carers on hydration in later life. Age Ageing 2019; 49(1): 111-118.

Braun J, Müller-Wieland D. Basislehrbuch Innere Medizin. Munich: Urban und Fischer; 2018.

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IQWiG health information is written with the aim of helping people understand the advantages and disadvantages of the main treatment options and health care services.

Because IQWiG is a German institute, some of the information provided here is specific to the German health care system. The suitability of any of the described options in an individual case can be determined by talking to a doctor. can provide support for talks with doctors and other medical professionals, but cannot replace them. We do not offer individual consultations.

Our information is based on the results of good-quality studies. It is written by a team of health care professionals, scientists and editors, and reviewed by external experts. You can find a detailed description of how our health information is produced and updated in our methods.

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Created on June 12, 2024

Next planned update: 2027


Institute for Quality and Efficiency in Health Care (IQWiG, Germany)

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