Not drinking enough: symptoms, warning signs and what to do

Headaches, dizziness, fatigue – sometimes there’s a simple reason behind them: you haven’t drunk enough. What symptoms do we experience when we don’t drink enough? A mild shortage often goes completely unnoticed. Here you can read about the typical signs, who is particularly at risk, the consequences of severe dehydration, and when you should seek medical advice. This article is not a substitute for medical advice.

Early signs: mild dehydration

In medical terms, a lack of fluids is known as ‘dehydration’. It can easily occur on hot days or after exercise, but can also be caused by vomiting, diarrhoea or blood loss. According to the Institute for Quality and Efficiency in Health Care (IQWiG), when the first signs appear, these often include, in addition to thirst:

• a dry mouth and a coated tongue
• Headaches
• Exhaustion and rapid fatigue
• a reduction in physical and mental capacity
• less frequent urge to urinate, as well as less urine and darker-coloured urine

The colour of urine is a simple indicator: very dilute urine is almost colourless, whilst concentrated urine is dark yellow. In older people, however, dark urine is a less reliable indicator.

Warning sign: if there is a noticeable lack of fluid

Moderate or severe dehydration may also manifest as dry mucous membranes, dizziness when standing up quickly, palpitations, abdominal and chest pain, sunken eyes and a lack of tears when crying, muscle cramps, seizures, lethargy or confusion. Confusion is one of the most important signs that dehydration has become significantly worse.

Emergency: Severe dehydration can lead to hypovolaemic shock – the circulatory system is then on the verge of collapse. It can also be recognised by cool hands and feet, restlessness, shivering or profuse sweating, as well as anxiety or apathy, and can even lead to loss of consciousness. Shock is a medical emergency. In hot weather, nausea, cramps, a high body temperature, very red or very pale skin, vomiting, confusion or circulatory collapse are also warning signs; in such cases, seek medical help or dial the emergency number 112 immediately.

Children and babies

If children are dehydrated, this is often evident in their behaviour: they are listless, restless, grumpy or tearful, but cry without tears, and sometimes breathe more quickly or deeply. In infants, symptoms include a sunken fontanelle and thirst-induced fever; babies and toddlers have significantly fewer wet nappies. Infants and toddlers tend to experience thirst as a feeling of unease and are often not yet able to say that they are thirsty.

According to the MSD Manual, dehydrated infants require immediate medical attention if the soft spots between the skull bones or the eyes are sunken, they do not produce tears when crying, their mouth is dry, they are passing little urine, or they appear listless.

Who is particularly at risk

• Older people: As people get older, many feel less thirsty. Some deliberately drink less – out of concern that they will have to go to the toilet frequently or experience involuntary urine leakage. Dementia or a lack of assistance with drinking also play a part, and often several factors come together, such as medical conditions and certain medicines. As the kidneys do not function as well in old age, older people should drink larger quantities spread over several hours, not exceeding 0.7 to 1 litre per hour.
• Children: When they have a fever or are exposed to heat, they lose more fluid than adults due to their relatively larger skin surface area, and gastrointestinal infections can lead to severe fluid loss in children.
• In hot weather and during exercise: excessive sweating increases your fluid requirements. You only start to feel thirsty once you have already lost a lot of fluid; in hot weather, the Federal Institute for Public Health (BIÖG) therefore recommends drinking a glass of water every hour, even if you are not thirsty.
• Illness and medication: diarrhoea, vomiting, fever, high blood sugar, kidney or endocrine disorders, and diuretics can lead to dehydration.
• In the hustle and bustle of everyday life: even when you’re thirsty, it’s easy to forget to drink.

Breastfeeding women have an increased need for fluids; whether pregnant women should drink more has not been studied.

The potential consequences of a significant shortage

Even mild dehydration can lead to mood swings and impair concentration and judgement. Severe dehydration can have serious consequences, particularly in older people. The IQWiG lists, amongst other things, falls and fractures, poorer tolerance of medication, constipation, infections (such as of the urinary tract), confusion, seizures, cardiac arrhythmias, low blood pressure, impaired kidney function, pressure ulcers, poorer wound healing and kidney stones. If dehydration persists, there is a risk of shock and severe damage to the kidneys, liver and brain.

As regards kidney stones, there is evidence to suggest that drinking more fluids may help prevent a recurrence: in one study conducted over five years, 27 out of 100 people who drank the same amount as before developed a second kidney stone – compared with 12 out of 100 among those who drank more.

What helps – and when to see a doctor

You can correct mild dehydration yourself by drinking more over the course of a few hours. After heavy sweating over a prolonged period, isotonic drinks containing electrolytes can be helpful; in the case of a fever or diarrhoea, for example, a salty broth. The article ‘Electrolytes no sugar’ explains what goes into an electrolyte drink.

If you experience unusual weakness, confusion or dizziness, it is important to seek medical advice or call an ambulance promptly – particularly in the case of children and older people. The same applies if you lose a lot of fluid over several days and are unable to replace it adequately, for example due to severe vomiting or excessive sweating. If the symptoms in children do not subside within a few days and they continue to lose a lot of fluid, it is important to seek advice from a paediatrician.

Anyone with heart or kidney failure should discuss their daily fluid intake with their doctor. In the case of certain conditions, such as heart or kidney failure or liver cirrhosis, a doctor may even prescribe a lower fluid intake.

How to make sure you drink enough in your day-to-day life

In Germany, the general guideline for adults is around 1.5 litres per day. You’ll need more during periods of intense physical exertion, in high temperatures, when taking certain medicines, during infections accompanied by a fever, when sweating, suffering from diarrhoea or vomiting, and whilst breastfeeding. The fluid intake calculator can help you work out your personal guideline.

• Keep an eye on your thirst – that’s the most important thing.
• Link drinking to specific times, such as during meals, whilst on the phone, when taking medication or after your lunch break.
• Prepare the daily portion in the morning in carafes or bottles – that way you can keep track of everything.
• Mix it up and make it look lovely: try different drinks, colourful glasses or pretty mugs. You can find ideas no sugar in ‘Make your own flavoured water’.
• Keep a record of how much you drink – keep a record of this for about a day, or log it in an app or on a fitness tracker.
• Eat foods with a high water content: melon, cucumber, tomatoes and strawberries help you stay hydrated on hot days.

In everyday life, water and unsweetened, non-alcoholic drinks such as fruit and herbal teas and juice spritzers with a little juice and plenty of water are best; tap water in Germany is safe to drink. You can find more information on this topic on the ‘Drinking Water’ page.

Frequently Asked Questions

Sources

1. Institute for Quality and Efficiency in Health Care (IQWiG), gesundheitsinformation.de: Dehydration, published 18 October 2023, next update 2026. gesundheitsinformation.de
2. Institute for Quality and Efficiency in Health Care (IQWiG), gesundheitsinformation.de: Drinking enough in everyday life: tips and advice, 18 October 2023. gesundheitsinformation.de
3. Institute for Quality and Efficiency in Health Care (IQWiG), gesundheitsinformation.de: Prevention of kidney stones, updated 22 February 2023. gesundheitsinformation.de
4. MSD Manual, Patient Edition: Dehydration, last updated January 2025. msdmanuals.com
5. MSD Manual, Patient Edition: Changes in the colour or smell of urine, last updated February 2026. msdmanuals.com
6. MSD Manual, Patient Edition: Dehydration in Children, last updated January 2025. msdmanuals.com
7. Federal Institute for Public Health (BIÖG), Climate – People – Health portal: Recommendations for hot weather, accessed 16 September 2026. klima-mensch-gesundheit.de
8. Federal Institute for Public Health (BIÖG), ‘Staying Active and Healthy as You Age’ portal: Drinking enough and drinking the right things: fluid intake and recommended drinks, accessed 16 September 2026. gesund-aktiv-aelter-werden.de
9. VerbraucherService Bayern im KDFB e. V.: Recommendations on fluid intake – a critical look, 25 August 2021. verbraucherservice-bayern.de