Image copyright Science Photo Library Image caption How much water is safe to drink?

Doctors often advise patients to "drink plenty of fluids" when unwell, but the case of a 59-year-old woman shows that drinking too much water too quickly can be dangerous.

She developed hyponatraemia or water intoxication after trying to stave off a urinary infection.

Doctors writing in BMJ Case Reports say this is a rare occurrence in otherwise healthy people.

But they say better guidance on fluid intake for patients is needed.

They write: "There is a paucity of research weighing the risks and benefits of advice to 'drink more fluids'."

Medical experts commenting on the case said it was important to keep drinking - but not excessively - if sweating or suffering from a fever and doctors should be more specific about how much water is safe to drink.

Hyponatraemia is a condition that occurs when the level of sodium - an electrolyte which helps control the amount of water in the cells - in the blood is abnormally low.

It is more often seen in endurance sports and after the use of ecstasy.

Losing control

The patient described was admitted to A&E in London for treatment for her infection.

The report describes how she soon became shaky, muddled, had major speech difficulties and vomited several times.

The patient recalled: "I remember seeing my hand in front of my shaking rather violently and I wondered why I could not stop it, then realised that my whole body was shaking.

Image copyright Science Photo Library Image caption Women need around 1600ml of fluid per day and men need 2000ml per day - but excessive drinking can be dangerous

"At that point I became terrified..."

She thought she was having a stroke and remembers being unable to express her feelings or control her movements.

When she revealed she had drunk several litres of water in a matter of hours, based on advice to try to flush out her system, doctors restricted her fluid intake over the next 24 hours and she recovered.

But she said she felt weak afterwards and it took her about a week to feel normal again.

In another previous case, a woman died after developing hyponatraemia from drinking excessive amounts of water when she had gastroenteritis.

How much water?

Tom Sanders, professor emeritus of nutrition and dietetics at King's College London, said the case report does not mean the advice on water is wrong.

"Patients should be provided with an adequate supply of water by their beds which they should be encouraged to drink or helped to drink."

He said this was especially important for elderly patients in hospital who often suffer from dehydration.

He added: "Water requirements are about 1ml/kcal energy which gives average fluid intakes of 2000ml for women and 2500ml for men."

Prof Graham McGeown, Dunville Professor of physiology at Queen's University Belfast, said: "Perhaps the lesson is that doctors need to be more specific in their guidance rather than avoiding advice on drinking in these circumstances."

Water essentials

Nearly two-thirds of the body is water

You get water about 20% of your total fluid intake from food

But women still need around 1600ml of fluid per day and men need 2000ml per day

If you are getting enough water, your urine should be a pale straw colour

If your urine is very pale, you may be drinking more than you need

Other drinks give you water too - such as tea, coffee, milk and juices

Source: British Nutrition Foundation