Researchers have come up with an unusual proposal to slow, or even reverse, the cognitive decline that comes with old age: small, daily doses of cannabis extract. The idea emerged from tests on mice which found that regular, low doses of tetrahydrocannabinol (THC) – the main psychoactive ingredient in cannabis – impaired memory and learning in young animals, but boosted the performance of old ones. The discovery has raised hopes for a treatment that improves brain function in old age without inducing the behavioural effects well known to recreational users of the drug.