Or so we thought.
A recently published story in Nature Medicine found that the cognitive functions of elderly mice actually improved after the administration of low doses of THC (the active ingredient in cannabis that makes everything either awesome or terrifying, depending on the user's disposition).
How? What? Why?
Basically, THC and CBD (another non psychotropic component in cannabis) contain biological compounds that mimic similar compounds in our brains called endogenous cannabinoids. That means your body is hardwired to receive small doses of cannabis (as compared to other substances — like alcohol — that alters your body’s chemistry the way a poison does).
When you use marijuana in excess, these naturally occurring cannabinoids can be overloaded, especially when you’re young and have more active neurological receptors in your brain. But as you age, they wane, so introducing cannabis into your system can actually replenish a supply that formerly occurred naturally.
In the study, which was conducted by Andreas Zimmer of the University of Bonn, younger mice performed poorly on a series of cognitive tests (they ranged from exiting water mazes to recognizing familiar mice) after exposure to low doses of THC. The older mice, though, performed worse when sober.
Zimmer's conclusion is that the younger mice had too many receptors, thus causing a cannabis overload that hindered brain activity. For the aging mice, the THC filled in the gaps between worn-out neurological receptors, giving them improved memory and cognition. This is consistent with other studies on aging by the same team.
Scientific American points out that “a little marijuana may alleviate anxiety but too much can bring on paranoid delusions. […] So the detrimental effects seen in young brains, in which cannabinoids are already plentiful, may turn out to be beneficial in older brains that have a dearth of them.”
Basically, THC might help aging brains maintain their natural chemical balace.
When used in moderation, of course.