In laboratory experiments, the compound, delta-9-tetrahydrocannabinol (THC), preserved levels of a brain chemical that declines in Alzheimer's, permitting the build-up of brain-gumming "amyloid plaques".
The plaques are the hallmark of Alzheimer's and its dementia-inducing damage.
"Our results provide a mechanism whereby the THC molecule can directly impact Alzheimer's disease pathology," researchers reported in the US journal Molecular Pharmaceutics.
The team - led by organic chemist Kim Janda of the Scripps Research Institute in La Jolla, California - claimed that THC holds real promise as a "drug lead", a model for developing new and more effective treatments for Alzheimer's.
Existing drugs such as donepezil, sold as Aricept in Australia, inhibited an enzyme called acetylcholinesterase which broke down acetylcholine, the brain chemical that prevents formation of amyloid plaques.
But THC not only acted against the enzyme, it also targeted plaque formation.
According to pathologist and Alzheimer's expert Colin Masters, the findings were novel and unsuspected.
"It might be possible to reformulate or rebuild the THC molecule so it has the anti-Alzheimer's effects without causing disturbances of cognition - getting high or stoned," said Professor Masters, from the University of Melbourne and the Mental Health Research Institute.
That's so because THC acts on one group of brain molecules when it triggers a buzz and another when it fights brain-clogging plaques.