Dehydration - Causes, Symptoms & Treatment

Dehydration is when the body loses more fluids than it takes in. It can be caused by various factors such as excessive sweating, not enough fluid intake or illnesses that lead to fluid loss. It’s more common during periods of hot weather or intense physical activity.

The symptoms of dehydration can vary in severity and may include things like a dry mouth, thirst, fatigue, dizziness, dark-coloured urine and reduced urination.

In more severe cases, confusion, a rapid heartbeat, low blood pressure and even fainting can occur.

The NHS recommends drinking at least six to eight glasses of fluids daily, which can include water, fruit juices or tea. Additionally, increasing fluid intake is advised when exercising or engaging in strenuous physical work to replace the fluids lost through sweat.

Certain groups of people are more vulnerable to dehydration, including older adults, young children and individuals with certain medical conditions. It’s essential for these individuals to take extra precautions and ensure they’re drinking enough water throughout the day.


It's important to note that the severity of dehydration symptoms can vary depending on the extent of dehydration. If you experience severe symptoms or are concerned, you should always seek medical attention.

The effects of dehydration include:

  • Feeling thirsty
  • Having a dry mouth
  • Sticky mouth and lips
  • Chapped lips
  • Fatigue
  • Dizziness
  • Weakness
  • Fainting
  • Headache
  • Dry skin
  • Muscle cramps
  • Confusion
  • Irritability
  • Dark-coloured urine
  • Urinating less often
  • Sunken eyes
  • Rapid heartbeat


Severe dehydration can have serious consequences for your health. When your body doesn’t have enough fluid, it struggles to perform vital functions properly.

If left untreated, it can lead to complications like heatstroke, where the body's temperature regulation fails and rises to dangerous levels.

Hypovolemic shock occurs when there isn't enough blood circulating in the body to supply the organs. Dehydration can also put strain on the kidneys, potentially leading to kidney damage or failure.

In severe cases, electrolyte imbalances can occur, increasing the risk of seizures. Additionally, inadequate fluid intake can create an environment for bacterial growth, increasing the likelihood of urinary tract infections (UTIs).

Stages of dehydration

There are three different stages of dehydration, with each representing varying levels of fluid loss in the body. Understanding these stages can help you to recognise the signs of dehydration and address it early.

Mild dehydration

The first stage is mild dehydration. At this stage, the body has lost a small amount of fluid, and any symptoms might not be noticeable immediately.

Common symptoms include a slightly dry mouth, increased thirst and possibly darker urine. You may feel slightly fatigued, but overall, you can prevent further dehydration by drinking more.

Moderate dehydration

The second stage is moderate dehydration. In this stage, the body has experienced a significant loss of fluids.

Symptoms become more pronounced, and you may experience a dry mouth, intense thirst, fatigue, dizziness and reduced urine output. Your urine may also appear much darker than usual.

If left untreated, moderate dehydration can lead to more severe complications, so it’s very important to drink fluids immediately.

Severe dehydration

The third stage is severe dehydration. At this stage, the body has lost a substantial amount of water, and the consequences can be severe.

Symptoms include extreme thirst, a dry and sticky mouth, rapid heartbeat, confusion, irritability, very dark urine or minimal urine output, sunken eyes, weakness and even fainting.

Severe dehydration requires immediate medical attention as it can lead to life-threatening conditions such as heatstroke or hypovolemic shock.


Dehydration causes include various things, but the main cause is often inadequate fluid intake. When we don't drink enough fluids, whether due to forgetting, lack of access to water, or simply not feeling thirsty, our bodies can become dehydrated.

Additionally, excessive sweating, which usually happens during intense physical activity or in hot weather, can cause significant fluid loss. Other causes include illnesses that result in symptoms such as fever and diarrhoea or vomiting, which can cause the body to lose water.

Certain medical conditions, such as diabetes or kidney disease, can contribute to dehydration, too.

Who is at risk

Certain groups of people will be more at risk of dehydration than others. Older adults, for example, are more susceptible due to changes in their body's ability to conserve water and sense thirst.

They may also have underlying health conditions or take medications that can make them more at risk.

Infants and young children are also at higher risk as their bodies have a higher proportion of water and are more prone to losing fluids through sweating, vomiting or diarrhoea.

Individuals who engage in intense physical activity or work in hot environments, such as athletes and outdoor labourers, are at increased risk due to excessive sweating and fluid loss.

Those with chronic illnesses such as diabetes, kidney disease or cystic fibrosis, may need more fluid intake. This also applies to pregnant women, especially during the later stages of pregnancy.


Diagnosing dehydration typically involves looking at a combination of symptoms, physical signs and medical history. A healthcare professional may start by asking about symptoms such as thirst, dry mouth, decreased urine output or fatigue.

They will also consider factors like exposure to heat or recent illnesses that may contribute to fluid loss. During a physical examination, they may check for signs like dry skin, low blood pressure, rapid heart rate or sunken eyes, which can indicate dehydration.

In some cases, urine tests may be done to assess the concentration of urine and if certain chemicals are present. Blood tests may also be performed to evaluate electrolyte imbalances, kidney function or other potential complications associated with dehydration.

These diagnostic tools help healthcare professionals determine the severity of dehydration and help them to decide on appropriate treatment.

When to call 111/an emergency doctor

Knowing when to seek medical assistance for dehydration is crucial to ensure timely and appropriate care. You must contact an emergency doctor, ask for an urgent GP appointment or get help from NHS 111 if you, your child or someone you know is experiencing:

  • Unusual tiredness or drowsiness
  • Confusion and disorientation
  • Dizziness upon standing that does not improve
  • Dark yellow urine or decreased urine output (or if your baby has fewer wet nappies)
  • Rapid breathing or a fast heart rate
  • Absence or minimal tears when crying (in the case of a baby or child)
  • Sunken fontanelle (the soft spot on a baby's head that sinks inwards)

These symptoms can indicate severe dehydration and require immediate treatment.


Home treatments

Home treatments for dehydration can be effective in milder cases. One of the most important steps is to drink plenty of fluids, such as water, clear broths or diluted fruit juices.

It’s best to avoid drinks that contain caffeine, alcohol or high amounts of sugar, as they can worsen dehydration.

Sucking on ice or ice lollies can provide hydration and help relieve dry mouth. Additionally, consuming watery fruits and vegetables, like watermelon or cucumber, can help.

Medical treatments


Rehydration sachets or tablets, such as O.R.S Hydration Tablets, are a helpful treatment for dehydration, especially when fluids and electrolytes need to be replaced.

These products contain a mixture of salts, sugars and minerals that help restore the body's hydration levels. They are dissolved in water and consumed orally.

They’re particularly beneficial in cases of vomiting, diarrhoea or excessive sweating, which can lead to fluid loss and electrolyte imbalance.

By providing the necessary electrolytes, such as sodium and potassium, along with fluids, these hydration products help the body absorb water more effectively.

Medical procedures

In more severe cases of dehydration, hospital treatment may involve intravenous (IV) fluids. This means that fluids and electrolytes are given directly into a vein through a small tube.

IV fluids are an effective way to rapidly replenish the body's water and electrolyte levels. They are especially necessary if fluid cannot be taken orally or when dehydration is severe.

This treatment is typically performed in a hospital under the supervision of healthcare professionals.


Preventing dehydration is essential for maintaining good health. To prevent dehydration, there are simple steps you can take. Firstly, make sure to drink plenty of fluids throughout the day, aiming for at least eight glasses of water.

In hot weather or during intense physical activity, increase your fluid intake to compensate for additional sweating and fluid loss. It's also beneficial to include hydrating foods in your diet, as we mentioned above.

These foods not only provide hydration but also contain important nutrients. Limiting caffeine and alcohol intake is important.

Monitoring the colour of your urine can also be helpful; pale yellow or clear urine indicates proper hydration, while dark yellow urine suggests dehydration.

During illness, such as episodes of vomiting, diarrhoea or fever, it's crucial to drink plenty of fluids to replenish what is lost.

Taking breaks and drinking water during physical activity is important to stay hydrated. By following these preventive measures and being mindful of your fluid intake, you can maintain proper hydration and reduce the risk of dehydration.

Remember, prevention is easier than treating dehydration once it occurs.

Alexandra Moses - Medical Content Writer
James O'Loan - CEO & Superintendent Pharmacist
James O'Loan , CEO & Superintendent Pharmacist on 04 June 2024
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