For decades, “drink plenty of fluids” has been standard medical advice for people suffering a long list of ailments, including the flu and gastrointestinal illnesses.

But a newly published report highlights the potential dangers of drinking too much water while dealing with something as simple and common as a urinary infection.

According to the case study outlined in the journal BMJ Case Reports, a 59-year-old woman in London became seriously ill with hyponatremia – a condition that causes abnormally low sodium levels – after drinking several litres of water in one day to help with her recurring urinary tract infection.

Doctors stress that it’s very rare for people with normal kidney function to develop hyponatremia, also referred to as water intoxication. The condition is most commonly observed in people with certain types of illnesses that drive up their anti-diuretic hormones, or in endurance athletes who end up consuming more water during exercise than their kidneys can process.

And of course, healthy people should continue to drink plenty of water and other fluids, as hydration improves a wide range of body functions.

But in the London woman’s case, drinking too much water landed her in hospital with serious symptoms.

She was admitted to the Royal London Hospital emergency department, where she became “progressively shaky, muddled, vomited several times, and had significant speech difficulties,” according to the report.

The woman’s urinary tract infection was being treated with antibiotics and painkillers. But tests revealed that her sodium level had dropped to 123 millimoles per litre. The normal range is between 135 and 145.

The woman told doctors that she drank several litres of water earlier that day, “to flush out her system.”

Water intoxication “is a medical emergency and requires prompt recognition and action,” the report says. BMJ notes that a mortality rate of nearly 30 per cent has been reported in patients whose sodium levels dropped to below 125 mmol/L.

The latest case, as well as another BMJ case report in which a woman with gastroenteritis drank “excessive amounts of water” and later died, highlight the need to reconsider fluid intake recommendations for certain types of patients, the report says.

The report says there is limited evidence behind the advice to drink plenty of fluids when it comes to managing “mild infective illness.”

“This needs to be addressed, especially considering the significant morbidity and mortality of acute hyponatremia."