Thirsty marathon runners are being warned not to drink too much water during this weekend's London marathon.
Paula Radcliffe won the London marathon last year
A study says those who take in too much fluid risk collapse, confusion or even death from excess water on the brain.
The Journal of Royal Society of Medicine study looked at 14 patients who developed exercise-associated hyponatraemia after the 2003 race.
Not only did the runners suffer some severe symptoms, many did not remember finishing the race, the UK study said.
Dr Dan Tunstall-Pedoe, medical director of the London Marathon who conducted the study, said that for some runners, symptoms had only developed after they finished the race.
The 14 were admitted to central London's St Thomas' Hospital, where 11 had symptoms of confusion.
Women also appeared to be at higher risk of the condition, with half of the patients being female, compared to about a quarter of runners over all.
The study said the "cluster of cases" was probably due to higher than usual temperature and the widespread promotion of drinking to prevent dehydration.
Not enough information on the risks of over-hydration was provided, it said.
The following year, when more detailed advice on safe drinking during the race was available, just one case of the condition was reported.
Dr Tunstall-Pedoe said: "One patient in the study estimated that he drank approximately 13 litres during the five hours he took to complete the marathon which is more than five times the recommended amount.
"Drinking excessively before during and after the event can be extremely dangerous."
Dr David Martin, an exercise psychologist from Georgia State University, who studied joggers' drinking habits believes people who take up running for the first time are often advised to drink too much water.
London Marathon organisers advise slower runners, or those taking longer than three hours 30 minutes to complete the course to drink no more than half a litre of fluid per hour.
This advice is particularly relevant at colder temperatures.
But faster runners may need as much as a litre of fluid per hour, they say.