Drinking tap or bottled water could help people suffering from low blood pressure who faint while standing up, scientists have said.
Researchers focused on 14 people with blood pressure problems
Researchers from Imperial College London found drinking two glasses of water (480ml) can raise blood pressure.
They had studied of 14 people whose bodies had problems regulating blood pressure and whose blood pressure fell when they stood up.
The Journal of Neurology, Neurosurgery and Psychiatry published the research.
The autonomic nervous system is responsible for the control of bodily functions not consciously directed, such as blood pressure, heart rate and sweating.
The researchers focused on 14 patients with autonomic failure. Seven had pure autonomic failure (PAF), a disorder affecting only the autonomic nervous system
The others had multiple system atrophy (MSA), a neurodegenerative disease marked by a combination of symptoms affecting movement, blood pressure and other body functions.
In each case the act of standing up caused a fall in blood pressure.
The patients were asked to drink 480ml of distilled water.
The PAF patients registered a significant rise in blood pressure five minutes after their drink.
For the MSA patients the effect was the same, but took 13 minutes to become apparent.
In both groups drinking water appeared to be enough to counter the fall in blood pressure caused by standing up.
Researcher Professor Christopher Mathias said: "People with low blood pressure caused by autonomic failure are at a greater risk of fainting when standing upright, after food or even after mild exertion.
"This can affect their life in many ways, stopping them from driving, or in extreme cases, from being able to work.
"This discovery could be of considerable use in helping these patients to understand why this happens.
"It may also benefit the many without autonomic failure who faint as a result of low blood pressure."
The Sarah Matheson Trust, which supports people with PAF and MSA, helped fund the research.
Alison Abery, a liaison nurse for the trust, said members would welcome the findings.
"It will be good for them to have something that helps to manage low blood pressure without the need to take on board extra medication," she said.