The advice to "drink plenty of fluids" when unwell lacks evidence, experts say.
Doctors at King's College Hospital in London questioned the recommendation after treating a 59-year-old woman who had drunk so much water that she became gravely ill.
The woman effectively overdosed on water after developing symptoms of a urinary tract infection, something she had suffered from before.
She recalled being told by a doctor previously to drink lots of water - half a pint every 30 minutes - though she said she thought in this case, she had consumed more to "flush out her system".
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.
It can occur if too much water is drunk over 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.
Writing in the journal BMJ Case Reports, doctors described how the woman got worse, explaining: "During her visit to the emergency department, she became progressively shaky and muddled. She vomited several times, was tremulous and exhibited significant speech difficulties."
Although doctors were able to save the woman's life with treatment including restricting her fluid intake to a litre over the next 24 hours, they described another case in which a young woman suffering from gastroenteritis died after consuming too much water.
People who take the drug MDMA and those taking part in endurance sports are also at risk of acute hyponatraemia.
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?
"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."
They concluded: "There is a paucity of evidence behind the advice to 'drink plenty of fluids' in the management of mild infective illness."
Dr Imran Rafi, chairman 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.
"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.
"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."