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Physical wellbeing
Hydration matters. But more isn't always better.
Most of us are told to drink more water, especially on long, busy days in the practice. That's good advice, up to a point. Very large amounts drunk quickly can overwhelm the body's ability to get rid of excess water and cause the sodium level in your blood to fall.
On this page
01 What happensHow too much water dilutes sodium in the blood. 02 How much is too much?What your kidneys can realistically clear. 03 Warning signsSymptoms to look out for, and when to call 999. 04 Who may be at higher riskSituations that make imbalance more likely.The short version: drink regularly, let thirst guide you, and don't force large volumes just to hit a number.
01 · What happens
Sodium helps control how much water sits inside and around your cells. When a very large amount of water is drunk in a short time, the sodium concentration in the blood can fall. This is called hyponatraemia.
If sodium falls quickly, water moves into cells. In severe cases this includes brain cells, which is why acute hyponatraemia can become a medical emergency.
Large amounts are drunk quickly
Intake outpaces the body's ability to remove the excess water.
Blood sodium becomes diluted
The balance between water and sodium in the blood shifts.
Water moves into cells
If the fall is rapid, cells swell, including in the brain.
02 · How much is too much?
Healthy kidneys can clear a lot of excess water, but only so fast. Medical literature commonly puts the maximum rate for a healthy adult at roughly 0.8 to 1.0 litres an hour.
Risk rises when intake goes beyond what the body can remove, especially when very large volumes go down in a short space of time.
This isn't a drinking target. Individual limits vary with kidney function, medication, illness, exercise and other factors.
litres per hour
Approximate rate at which healthy kidneys can excrete water.
Rough guide, not a target03 · Warning signs
Symptoms depend on how quickly sodium levels fall and how far. Early signs can easily be mistaken for tiredness or dehydration.
Headache
Nausea or vomiting
Confusion
Muscle cramps or weakness
Drowsiness
Seizures or loss of consciousness are emergencies
Severe confusion, a seizure, collapse or loss of consciousness needs urgent medical attention. Call 999 if someone is seriously unwell or unconscious.
04 · Who may be at higher risk?
Endurance athletes
Particularly when large volumes are drunk during long events such as marathons.
People taking certain medicines
Some medicines affect sodium levels or how the body handles water.
People with kidney, heart or liver problems
These conditions can affect the body's ability to regulate fluid.
Anyone drinking a lot very quickly
Speed matters, because the kidneys need time to clear excess water.
of tested runners had hyponatraemia after the race
A useful data point
A study of 488 runners at the 2002 Boston Marathon found 13% had hyponatraemia after the race, and 0.6% had critical hyponatraemia.
Weight gain during the race was strongly linked to the condition, a sign that runners had taken in more fluid than they were losing.
Worth knowing: choosing sports drinks over water didn't remove the risk. Overdrinking was the problem, whatever was in the cup.
Free poster
Everything on this page is also available as a one-page poster. Download it free and print it for the staff room, kitchen or a clinical area, so the message is there when people need it.
You'll find it alongside our other wellbeing posters on the downloads page.
The simple message
6–8 glasses of fluid a day is the NHS general guide for most people. If you've been advised to restrict fluids, follow your healthcare team's advice.
Evidence
Hydration is one part of looking after yourself at work. Find more practical ideas for movement, breaks and eating well in the Physical Health hub.
This page is general information and wellbeing education, not individual medical advice. If you're worried about symptoms or how much you're drinking, speak to a healthcare professional. Seek urgent medical attention for severe confusion, seizures, collapse or loss of consciousness.
