Overhydration Symptoms: 9 Signs of Drinking Too Much Water

Overhydration, also called water intoxication, is drinking more water than your kidneys can clear, so the extra water dilutes the sodium in your blood.¹ The resulting low blood sodium (hyponatraemia) can cause headache, nausea and confusion, and in severe cases seizures and coma.¹ Vomiting and breathing difficulty are also reported in moderate to severe cases in endurance athletes.⁷ Sudden confusion or severe difficulty breathing needs a 999 call.³ A first seizure also needs a 999 call.⁴

Understanding Overhydration

Water intoxication is thought to be very rare in the general population because the kidneys can clear excess water quickly, but it can happen when someone drinks more than the kidneys can clear, or when something reduces how much water they can clear.¹

What is Overhydration?

Overhydration, as the name suggests, is when you consume more water than your body can eliminate. When you take in excess water, it can dilute the sodium in your blood.¹ This condition is known as hyponatraemia, which is defined as a blood sodium below 135 mmol/L.² Symptoms range from subtle to severe, and can be life-threatening.²

Causes of Overhydration

There are several reasons why someone might become overhydrated. Here are the main ones reported:

• Excessive fluid intake: This is drinking large amounts of water faster than the kidneys can clear it, for example in a water-drinking competition, in the belief that it is good for health, or because of a compulsion to drink.¹
• Certain psychiatric conditions: Psychiatric conditions that involve compulsive water drinking, known as psychogenic polydipsia, were the most common background in one systematic review of water intoxication case reports and case series.¹

Risk Factors for Overhydration

Overhydration was also reported in people with no underlying condition (31% of case reports and case series in one systematic review).¹ Certain factors raise the risk:

• Endurance athletes: Marathon runners, ultramarathon runners and triathletes can drink more than they lose in sweat during long events, and drinking beyond thirst is the main driver of exercise-associated hyponatraemia.⁶
• People with certain health conditions: Chronic psychiatric disorders were present in 52% of the case reports and case series in a systematic review of water intoxication.¹
• Certain medications and drugs: Thiazide diuretics (water tablets), antipsychotic medicines and the recreational drug MDMA can reduce the kidneys' ability to clear water, so less water is needed to cause harm.¹

The 9 Signs of Overhydration

The signs of overhydration come from low blood sodium, so they run from subtle to severe.² Other conditions, including dehydration and heat exhaustion, can cause similar signs.⁷ Most published evidence on water intoxication comes from case reports of severe cases.¹ The milder signs below are mostly described in endurance athletes.⁷ Here are nine signs to watch out for:

Swelling or Puffiness

Swelling or puffiness, often in the hands or feet, is one of the most commonly reported mild signs of low blood sodium in endurance athletes.⁷ Swelling from fluid collecting in the ankles, feet or legs is called oedema, and it has many other causes, including standing or sitting for too long, salty food, some medicines, and problems with the kidneys, liver or heart.⁵ The NHS says to get urgent advice from a GP or NHS 111 if swelling is severe, painful or starts very suddenly.⁵

Headaches

Headache is one of the most commonly reported mild signs of low blood sodium in endurance athletes.⁷ In exercise-associated hyponatraemia, water may shift into the confined space of the skull, and in advanced cases this can develop into hyponatraemic encephalopathy, a serious brain complication.⁷ Headache alone is not a reliable guide: in a study of marathon runners treated for complaints such as headache and vomiting, no single sign separated runners with low blood sodium from those with normal levels.⁷

Nausea and Vomiting

Nausea is one of the most commonly reported mild signs of exercise-associated hyponatraemia.⁷ Vomiting is among the most common signs of moderate to severe exercise-associated hyponatraemia.⁷ If it follows drinking a lot of water and you are unsure what is wrong, NHS 111 can direct you to the right service.¹¹

Confusion or Disorientation

Confusion or disorientation is one of the most common signs of moderate to severe low blood sodium in athletes.⁷ The NHS says to go to A&E or call 999 if someone suddenly becomes confused.⁸

Seizures

Seizures are classed as a severe feature of water intoxication.¹ In a systematic review of 223 case reports and case series (590 people), 53% described severe features (seizures or coma) and 13% reported a death (not all of them attributed to the water), although most of the reports described severe cases, so the figures do not show how common seizures are.¹ The NHS says to call 999 if someone has a seizure for the first time.⁴

Difficulty Breathing

Difficulty breathing can be a sign of severe overhydration.⁷ Severe difficulty breathing is an emergency: the NHS says to call 999 or go to A&E immediately if you are gasping, choking or not able to get words out, or your chest feels tight or heavy.³ In advanced cases of exercise-associated hyponatraemia, fluid can collect in the lungs (non-cardiogenic pulmonary oedema, which happens without heart failure), and chest tightness, breathlessness and frothy sputum are among the reported signs.⁷ In a case series of seven healthy marathon runners who collapsed and were admitted with pulmonary oedema, the average blood sodium was 121 mmol/L, oxygen saturation was reported as below 70%, all needed a breathing tube and a ventilator, and one died.⁹ These were severe cases from a marathon. For milder breathlessness, the NHS says to contact NHS 111.³

Muscle Weakness, Cramps, or Spasms

Weakness or lethargy is one of the most commonly reported mild signs of low blood sodium in athletes, and muscle pain, aches or cramps are also reported.⁷ Drinking beyond thirst during exercise has not been shown to reduce muscle cramping.⁶

Weight Gain

During exercise, weight gain is a sign that fluid intake has been greater than fluid losses, and people who gain weight should reduce their fluid intake.⁶

Decreased Urine Output or Clear Urine

The NHS says most people should aim to drink enough during the day for their pee to be a clear pale yellow colour, so pale pee on its own is the usual target.¹⁰ In some athletes with low blood sodium, the hormone vasopressin is not switched off as it should be when the blood is dilute, so the kidneys hold on to water and excrete less of it.⁶

How Much Water Is Too Much?

NHS guidance, taken from the government's Eatwell Guide, is to aim for 6 to 8 cups or glasses of fluid a day, and it is a guide: you may need more if you are pregnant or breastfeeding, in a hot environment, physically active for long periods, or ill or recovering from illness.¹⁰ Kidneys can clear about 0.8 to 1.0 litres of water an hour, and overhydration happens when intake outpaces that.¹ For exercise, a review of the evidence recommends drinking when thirsty rather than to a fixed amount, because fluid losses vary between people and conditions.⁶ The amount needed to cause harm varies and is lower for some people: certain medicines, MDMA and smoking were reported to reduce it by up to about a third.¹ The authors of the systematic review of water intoxication note that guidelines do not set a safe upper limit for water intake, because needs vary widely.¹

Tips to Prevent Overhydration

Not drinking excessive fluids is the main strategy against exercise-associated hyponatraemia, and the evidence supports drinking to thirst during exercise.⁶

Monitor Your Daily Fluid Intake

Check your usual intake against the NHS guide, because habitual drinking in the belief that water has health benefits accounted for 7% of the case reports and case series in one systematic review of water intoxication.¹ Tea, coffee, lower-fat milk and lower-sugar drinks all count towards your daily fluid intake.¹⁰

Listen to Your Body’s Thirst Cues

For exercise, the recommended approach is to drink when thirsty before, during and immediately after, because fixed fluid targets do not suit everyone's sweat and urine losses.⁶ The review's conclusion applies to exercise in temperate climates lasting less than 17 hours, where it says drinking to thirst will prevent exercise-associated hyponatraemia.⁶

Adjust Fluid Intake During Exercise and Hot Weather

The NHS says you may need more fluid in a hot environment or when active for long periods, and that water is the best way to replace fluids lost through sweating.¹⁰ More is not better, though: sodium-containing sports drinks will not prevent exercise-associated hyponatraemia in athletes who overdrink during exercise, because it is the amount of fluid, not the amount of sodium, that sets the final blood sodium level.⁶

Talk to a GP or Pharmacist if You Are at Risk for Overhydration

If you take a medicine or have a condition that raises the risk of overhydration, ask a GP or pharmacist how much you should drink.¹ In 13% of the case reports and case series in one systematic review, the water was drunk on medical advice (for example before an ultrasound), and the authors identified misreading of that advice, such as "as much as you can", as the key factor, so ask for a specific amount if you are told to drink more.¹

When to Seek Medical Help

Call 999 or go to A&E for sudden confusion, a first or prolonged seizure, or severe difficulty breathing. For anything less urgent, NHS 111 can direct you to the right service.¹¹

• Sudden confusion: the NHS says to go to A&E or call 999 if someone suddenly becomes confused.⁸
• A seizure: call 999 if it is the first time, if it lasts longer than is usual for that person (or longer than 5 minutes if you do not know), if they do not recover as usual afterwards, or if they have another seizure without recovering in between.⁴
• Severe difficulty breathing: call 999 or go to A&E immediately if you are gasping, choking or not able to get words out, or your chest feels tight or heavy.³

If you are unsure how serious symptoms such as headache and nausea are after drinking a lot of water, NHS 111 can direct you to the right service, including a pharmacist or an out-of-hours GP.¹¹

Sources

1. Rangan GK et al. Clinical characteristics and outcomes of hyponatraemia associated with oral water intake in adults: a systematic review. BMJ Open, 2021. PMC8663108
2. Spasovski G et al. Clinical practice guideline on diagnosis and treatment of hyponatraemia. European Journal of Endocrinology, 2014. European guideline on hyponatraemia
3. NHS. Shortness of breath. nhs.uk/conditions/shortness-of-breath
4. NHS. Epilepsy. nhs.uk/conditions/epilepsy
5. NHS. Swollen ankles, feet and legs (oedema). nhs.uk/conditions/oedema
6. Hew-Butler T, Loi V, Pani A, Rosner MH. Exercise-associated hyponatremia: 2017 update. Frontiers in Medicine, 2017. PMC5334560
7. Armstrong LE, McDermott BP, Young SL, Casa DJ. Exercise-associated hyponatremia: serum sodium, symptomatology, severity, and sport specificity. Open Access Journal of Sports Medicine, 2025. PMC12630018
8. NHS. Sudden confusion (delirium). nhs.uk/conditions/confusion
9. Ayus JC, Varon J, Arieff AI. Hyponatremia, cerebral edema, and noncardiogenic pulmonary edema in marathon runners. Annals of Internal Medicine, 2000. PubMed 10787364
10. NHS. Water, drinks and hydration. nhs.uk water, drinks and nutrition
11. NHS. When to use NHS 111 online or call 111. nhs.uk when to use 111