The time-honoured medical advice to drink plenty of fluids when unwell could actually be harmful, experts have warned.
Doctors at King's College Hospital in London said the advice lacks evidence, after they treated a 59-year-old woman who drank so much water that she became seriously ill.
The woman effectively overdosed on water after developing symptoms of a urinary tract infection, which she had previously suffered from.
She said she consumed large quantities of water to "flush out her system" after being previously told by a doctor to drink half a pint every 30 minutes.
The woman was admitted to A&E, where doctors found she was suffering from acute hyponatraemia, which is caused by low salt levels in the blood.
The condition can occur if someone drinks too much water in a short period of time. Symptoms include nausea, vomiting and headaches.
In serious cases, the brain can swell, which can lead to confusion, seizures, coma and death.
A death rate of almost 30% has been reported in patients with abnormally low salt levels.
Doctors were eventually able to save the woman's life with treatment, including restricting her fluid intake to a litre over the next 24 hours.
Writing in the journal BMJ Case Reports, the doctors said: "We frequently advise our patients to 'drink plenty of fluids' and 'keep well-hydrated' when they are unwell.
"But, what do we mean by that? Are there potential risks of this apparently harmless advice?
They added: "As demonstrated here, the harmful effects of increased fluid intake include confusion, vomiting and speech disturbance, and potential for catastrophic outcomes due to low blood sodium concentrations."
Dr Imran Rafi, chairman of clinical innovation and research at the Royal College of GPs, said: "We would encourage patients to drink more if they have symptoms of dehydration, such as feeling thirsty - including in hot weather or when exercising - or passing dark-coloured urine.
"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."
He added: "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."