It’s a frequent recommendation by doctors when a patient is ill: Drink plenty of water. But health experts have warned not to take the long-established advice too literally, as doctors recently treated a patient who drank so much water that she became seriously ill.

When ill, you are at an increased risk of becoming dehydrated, and so need to compensate by drinking more water in order to aid recovery. Yet in a case highlighted by medics at King’s College hospital, and reported in BMJ Case Reports, it seems a 59-year-old patient went a little overboard and overdosed on water.

“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,” explained Dr Maryann Noronha, co-author of the case report, to The Guardian. “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.”

Suffering from a urinary tract infection, the woman drank plenty of water to “flush out her system,” consuming a previously recommended half pint every 30 minutes. Yet rather than helping her, the vast amounts of liquid instead meant she was admitted to the hospital with acute hyponatremia, caused by incredibly low sodium (salt) levels in her blood. This happens if a patient’s blood levels fall under 134 mmol/L.

Doctors recorded the woman’s blood sodium levels at a dangerously low 123 mmol/L, while a healthy person should have levels in the region of 135 to 147 mmol/L. This basically means that the blood was so dilute, extra water started to enter the cells of the body. This can also occur when people who are exercising hydrate too much, or even when people taking drugs such as ecstasy try to overcompensate for the amount they are sweating. It can then result in the potentially fatal swelling of tissue, especially if it occurs in the brain.  

With normal renal function, the authors note, it's difficult to overdose on water. However, those with certain illnesses can develop increased levels of antidiuretic hormones, which in turn reduces their excretion of water.

The woman with the urinary tract infection, who eventually recovered, was not the only case mentioned in the report. The authors also mentioned a patient with gastroenteritis who developed hyponatremia, resulting in death. These cases, they suggest, mean that doctors should highlight “the need to qualify our advice regarding water consumption in simple infective illness.” While it's important to stay hydrated when ill, patients should be careful when that's due to infection, as they risk further complications.