Low doses of the active ingredient in cannabis, THC, may reverse brain ageing and restore learning and memory, at least according to studies of older mice (New Scientist, 2017). In some cultures it’s traditional for older people to smoke cannabis, which is said to help them pass on tribal knowledge.

The research was led by Andreas Zimmer at the University of Bonn, who said “We repeated these experiments many times. It’s a very robust and profound effect.”

Andreas Zimmer’s team has been studying the mammalian endocannabinoid system, which is involved in balancing out the bodies’ response to stress. THC has the affect of mimicking similar molecules in this system, which calms people down. The researchers discovered that mice with genetic mutations that stop this endocannabinoid system from working properly age faster than normal mice, and show more cognitive decline.

This made Andreas Zimmer wonder if stimulating the endocannabinoid system in elderly mice might have the opposite effect. To find out, the team gave young (two month old), middle aged (twelve month old) and older (eighteen month old) mice a steady dose of THC. The amount they received was too small to give them psychoactive effects.

After a month, the team tested the mice’s ability to perform cognitive tasks, such as finding their way around mazes or recognising other individuals. In the control groups, which received no THC, the young mice performed far better than the middle aged and elderly mice. But the middle aged and older mice who had been given THC performed as well as the young mice in the control group. Further studies showed that THC boosted the number of connections between brain cells in the hippocampus, which is involved in memory formation. Andreas Zimmer said “It’s a quite striking finding.”

THC seemed to have the opposite effect in young mice. When they were given THC, their performance in some tasks declined. Young humans also perform worse in learning and memory tests in the hours and days after smoking cannabis, but a spliff delivers far higher doses than the mice received. Claims that heavy cannabis use can permanently impair cognition are disputed.

Andreas Zimmer thinks his findings show that both too much and too little stimulation is harmful. The endocannabinoid system is most active in young mice and humans, so extra THC may overstimulate it. In older mice, by contrast, endocannabinoid activity declines, so a little THC restores it to optimum levels.

The team’s findings aren’t that surprising according to neuropsychopharmacologist David Nutt of Imperial College London. Animal studies have shown that the cannabinoids the body produces itself can have beneficial effects on the brain. David Nutt and his colleagues have also found that THC use protects alcoholics from alcohol-induced brain damage.

Andreas Zimmer’s team is now planning human trials to find out whether older humans can benefit from low doses of THC too and, if so, from what age it is beneficial. He said “There is no formula to translate mouse months into human years.”

The trials will use purified THC rather than weed so the dosage can be precisely controlled. It might be administered as a mouth spray, for example.

Even if the trials get similar results, it is unlikely that doctors will start prescribing spliffs to older people. Andreas Zimmer said “The dosing is important. Smoking marijuana is very different.”