Tests of the drug rivastigmine - marketed as Exelon - found it was effective at treating some symptoms of mild or moderate Alzheimer's.

Additionally, the effects appeared to improve as the dose increased.

However, at higher doses there were slight gastrointestinal side-effects.

Progress towards better treatment

The findings are published in the British Medical Journal and bring doctors a step closer to understanding the workings of Alzheimer's.

The disease is the most common form of senile dementia, affecting up to 10% of adults over 65 and 50% of those over 80.

It causes the brain's functioning to deteriorate, and is characterised by a build-up of plaques in the brain.

These drugs aim to restore the chemical balance of the brain

Rivastigmine is one of a group of drugs, Acetylcholinesterase inhibitors, that are designed to restore the chemical imbalance that occurs in Alzheimer's patients' brains.

So far they have been the most successful treatments developed. Tacrine and donepezil are among them.

"This is the first treatment to show compelling evidence of efficacy," said Dr Michael Rosler, who led the study at Psychiatrische Universitatsklinik in Wurzburg, Germany.

He found the main benefit of the drug was that it enabled patients to perform daily activities.

This reduces dependence on carers and prolongs the time patients can live in their own homes.

'Completing the jigsaw'

The patients were all aged between 50 and 85. They were assigned at random to take the drug or a placebo.

More than a third of patients taking 6-12 mg of the drug per day improved over a six-month period.

Dr Tony Bayer of the University of Wales College of Medicine commented on the research.

He said it provides another piece in the Alzheimer's jigsaw, but says there are some shortcomings in their study.

The 725 patients who participated in the trial were not typical of those generally seen by psychiatrists and geriatricians, he said.