People with the eye condition glaucoma should avoid drinking large volumes quickly, say Auckland University researchers.
Their study found that drinking a litre of water in 15 minutes boosted pressure within the eye by 12.5 to 56 per cent.
High internal eye pressure is the cause of glaucoma, which results in irreversible damage to the optic nerve, the conduit of visual information from the eye to the brain.
Glaucoma, New Zealand's leading cause of preventable blindness, affects 2 per cent of 40-year-olds but is primarily a disease of the elderly, affecting 10 per cent of those aged 70.
Associate Professor Helen Danesh-Meyer, who led the study, said that because of public awareness campaigns the disease was being picked up earlier.
"On the basis of this study, it is reasonable to recommend that patients with glaucoma should not down a litre of water quickly - or any other liquid," she said.
"This has particular ramifications for people who may be involved in sports - and also people who go to the pub and drink a lot of fluid.
"I have one patient who is a cyclist. We have had to advise him to rehydrate at a slower pace after exercise."
Large and frequent spikes in eye pressure, such as those associated with drinking large volumes fast, could precipitate the development of full glaucoma in those who were borderline.
The study compared patients treated by eyedrops with those given surgery to relieve eye pressure. The average maximum pressure reached in the surgical group was 12.5 per cent above the baseline, and for the medically treated group it was 56 per cent.
Glaucoma New Zealand chairman Dr Ken Tarr said glaucoma patients often asked what they could do to help themselves.
Apart from taking their medication, the only practical advice was limiting the speed at which they consumed large volumes of liquid.
* Glaucoma NZ recommends everyone have their eyes tested for the condition by the age of 45 and every five years after that, or more frequently if there is a family history of the disease or other risk factors.
Tests at an optometrist cost about $90 in Auckland.
Dr Tarr said people who could not afford this could be referred to a public hospital by a GP.