The drug works on nerve cells in the brain

Zolpidem is usually used to treat insomnia.

South African researchers, writing in the NeuroRehabilitation, looked at the effects on three patients of using the drug for up to six years.

But one expert in neurological rehabilitation said it was possible the patients had a different condition.

This drug could be unmasking a condition from which people are able to wake up temporarily

Professor Mike Barnes, Hunters Moor Rehabilitation Centre

A person in a vegetative state will appear to be awake and may have their eyes open, but will show no awareness of their surroundings.

They will not be able to interact with other people, and will show no responses to sounds or things that happen around them.

But they will show signs of movement, and cycles of sleep and may be able to breathe on their own.

Response

Each of the three patients studied was given the drug every morning.

An improvement was seen within 20 minutes of taking the drug and wore off after four hours, when the patients restored to their permanent vegetative state.

Patient L had been in a vegetative state for three years, showing no response to touch and no reaction to his family.

After he was given Zolpidem, he was able to talk to them, answering simple questions.

Patient G was also able to answer simple questions and catch a basketball.

Patient N had been "constantly screaming", but stopped after being given the drug when he started watching TV and responding to his family.

Dr Ralf Clauss, now in the nuclear medicine department at the Royal Surrey Hospital was one of the researchers who carried out the study.

He told the BBC: "For every damaged area of the brain, there is a dormant area, which seems to be a sort of protective mechanism.

"The damaged tissue is dead, there's nothing you can do.

"But it's the dormant areas which 'wake up'."

Activity trigger

He said drugs like Zolpidem activate receptors for a chemical called GABA in nerve cells in the brain.

When brain damage occurs, these receptors appear to change shape, so they cannot behave as normal.

He said the drug appeared to cause the receptors in these dormant areas to change back to their normal shape, triggering nerve cell activity.

Dr Clauss added: "We are carrying out further research. The next step is to get rid of the sleepiness effect of the drug."

Mike Barnes, professor of neurological rehabilitation at the Hunters Moor centre in Newcastle, said it was possible that the patients had not had "true" PVS.

"A diagnosis of PVS means the patient should not wake up and respond."

He said a study carried out by specialists 10 years ago had shown that up to 45% of patients diagnosed with PVS actually had a range of different conditions, from which they could wake up.

Professor Barnes added: "This drug could be unmasking a condition from which people are able to wake up temporarily.

"But if they did have PVS, it would be a remarkable finding, and certainly worth further research."