Cannabis treatment could slow down - or even reverse - memory loss and other brain ageing effects in older people, say scientists.
New research on mice has shown that low doses of THC, the main "high" inducing chemical in cannabis, can put the brakes on mental decline.
The benefits were only seen in older mice, said the researchers.
When young animals were exposed to THC, their memory and learning performance got worse.
Although the different outcomes highlight the complexity of the drug's effects on the brain, cannabis has great potential for tackling some of the problems of ageing, the authors claim.
The team led by Dr Andreas Zimmer, from the University of Bonn in Germany, wrote in the journal Nature Medicine: "Cannabis preparations and THC are used for medicinal purposes.
"They have an excellent safety record and do not produce adverse side-effects when administered at a low dose to older individuals.
"Thus, chronic, low-dose treatment with THC or cannabis extracts could be a potential strategy to slow down or even to reverse cognitive decline in the elderly."
British experts described the research as "exciting" and "robust".
Former government drugs tsar Professor David Nutt, from Imperial College London, said testing the same approach in humans would not be possible in the UK because of "ridiculous" legal restrictions.
He added: "However, I think cannabis might be neuroprotective in humans as our own research has shown that alcoholics who use cannabis have less alcohol-induced brain damage than those who don't."
The German team administered low doses of THC to the mice through tiny implantable pumps.
Learning and memory tasks were used to test the mental performance of young (two month old), mature (year old) and elderly (18 month old) animals.
Learning and memory were improved by THC treatment in mature and old mice, but the reverse effect was seen in young rodents.
In older animals, exposure to the cannabis chemical appeared to turn back the clock in the hippocampus, the brain's key memory centre.
Gene activity in the brain region was restored to levels normally seen in young mice.
In addition levels of synaptic proteins, involved in nerve signalling, were increased and dendritic spines - small cell protrusions vital to neural connectivity - became more dense.
Dr Michael Bloomfield, clinical lecturer in psychiatry at University College London, said: "What is particularly exciting about this research is that it opens up a whole new chemical system, called the endocannabinoid system, as a potential target for new avenues of research which could include illnesses like dementia.
"However, we are still in very early days and further research is needed ... the possibility of doctors potentially prescribing, cannabis THC or similar compounds for memory problems in older people is still a long way off."
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