The common advice to drink plenty of water when ill is based on scant evidence and can actively harm chances of recovery, doctors have warned.

Medics at King’s College hospital NHS foundation trust, in London, raised the alarm after they treated a patient with hyponatremia – abnormally low sodium – from drinking too much water to help with a recurring urinary tract infection.



In the case highlighted, a 59-year-old woman consumed several litres of water based on medical advice she recalled from previous similar episodes to “flush out her system”. She became progressively shaky, muddled, vomited several times and had significant speech difficulties.

Dr Maryann Noronha, the co-author of the study published in BMJ Case Reports on Thursday, said: “When people are ill they don’t tend to drink very much water because it’s the last thing they want to do and you can become dehydrated very quickly.

“To counteract that risk, doctors have said ‘Make sure you drink lots of water.’ That has perpetrated the myth that you must drink gallons of water. Most people don’t do that but in this case they did it to the letter.”

Tests revealed her high intake of water had resulted in dangerously low sodium levels – 123 mmol/L – classifying it as a medical emergency. A mortality rate of almost 30% has been reported for patients with sodium levels of less than 125 mmol/L.



Doctors restricted her fluid intake to 1 litre over the next 24 hours. By the following morning her blood tests were normal and she was discharged later that day.

In a previous case a woman with gastroenteritis developed hyponatremia and died from drinking excessive amounts of water. Fatal water intoxication has also been reported in people engaged in endurance exercise and using the drug MDMA (ecstasy), when they have sweated heavily and overcompensated with fluids.

The authors stress that it is rare to develop water intoxication with normal renal function but warn that some illnesses drive up levels of antidiuretic hormones, which reduce normal excretion of water.

“Doctors should try to be more specific in their advice,” said Noronha. “I say to people, while they are ill they should at least consume their normal fluid intake and up to half again [ie, up to 150%]. If you drink three litres, you shouldn’t drink six litres when you are ill.”

Public Health England recommends people should drink six to eight glasses of fluid a day, including water, lower fat milk and sugar-free drinks including tea and coffee.

Noronha said the amount needed by different people varies, but the main message is not to change your consumption too greatly when you are ill. “If you are someone who doesn’t drink much water and then suddenly fill your body with masses that’s going to have a very big effect,” she said. She hopes the paper might prompt research in the area so that objective guidance can be drawn up.

Dr Imran Rafi, the chair of clinical innovation and research at the Royal College of GPs, said: “Drinking enough water is important in keeping healthy, both physically and mentally, and patients should keep their fluids up when unwell, particularly in conditions that can cause dehydration.



“There is no steadfast recommendation as to how much water people should drink in order to stay healthy, but the key thing is to keep hydrated – and passing clear urine is a good indication of this.

“This case report highlights that excessive water intake can have important consequences for patients, and this is something that healthcare professionals, and patients, should be mindful of.”

What is the advice?