Alzheimer's might partly develop because of the suppression of "natural cannabis" molecules in the brain, scientists believe.
Researchers in the US linked early symptoms of the disease to losing the beneficial effects of endocannabinoids.
The signalling molecules are natural versions of psychoactive chemicals in cannabis.
A rogue protein called amyloid-beta, suspected of playing a key role in Alzheimer's, is believed to block endocannabinoids in the brain in the earliest stages of the disease.
Endocannabinoids are part of the process that allows important signals in the brain to shine through while unwanted "noise" is shut out.
Blocking them results in the brain becoming too inhibited, leading to impaired learning and memory loss.
The scientists from Stanford University in California, who report their findings in the journal Neuron, warned that simply smoking marijuana was not a solution to Alzheimer's.
Senior author Dr Daniel Madison said: "Endocannabinoids in the brain are very transient and act only when important inputs come in. Exposure to marijuana over minutes or hours is different: more like enhancing everything indiscriminately, so you lose the filtering effect. It's like listening to five radio stations at once."
Flooding the brain with external cannabinoids also ran the risk of inducing tolerance and impeding the ability of natural endocannabinoids to do their job, he said.
Pinning down the molecular details behind amyloid-beta's effect on endocannabinoids could pave the way to new Alzheimer's drugs, said the researchers.