Scientists at one of Spain's leading research centres claimed yesterday to have found evidence that cannabis helps prevent the memory loss experienced by people suffering from Alzheimer's.

The potential breakthrough in understanding a disease that affects nearly half a million people in Britain, and around nine million worldwide, was made by a team led by María de Ceballos at the Cajal Institute in Madrid.

Their study seems to show that THC, the main active ingredient in cannabis, inhibits the activity of cells that cause damage to neurons in the brain.

Although the study is preliminary, it was welcomed by patient groups.

"Right now, there are no good drugs for Alzheimer's. There are some that treat symptoms but nothing that halts the disease," said Susanne Sorensen, head of research at the Alzheimer's Society.

While the beneficial effects of cannabis looked promising, Dr Sorensen cautioned that people with Alzheimer's should not start using the drug to help their memories, because of side effects.

Memory loss in Alzheimer's patients is not fully understood, but part of the problem is thought to lie with cells called microglia that surround neurons in the brain. In Alzheimer's, the activity of microglia gets out of control, damaging neurons and killing off parts of the brain. Dr de Ceballos's team conducted two separate experiments using human brain tissue and rats which showed that THC inhibits the activity of microglia, thus reducing memory loss.

Dr de Ceballos said the results showed that THC could help prevent memory loss in Alzheimer's patients, although the reasons why this might happen are still to be explored.

The next stage, she said, would be to test the rats using a synthetic equivalent of THC which inhibits the activity of microglia without intoxicating the rats as well.

"There's a long way to go before we will know if it is actually possible to stop the progression of Alzheimer's," she said.