All it takes is smoking the equivalent of a single 'spliff' of cannabis to make people less willing to work hard, finds a new UCL study.
The study claims to be the first to reliably demonstrate the short-term effects of marijuana on human motivation.
The lead author Dr Will Lawn, UCL Clinical Psychopharmacology, said that cannabis has been commonly thought to reduce motivation, but this is the first time it has been reliably tested and quantified using an appropriate sample size and methodology.
The first part of the research involved 17 adults who all used cannabis occasionally. Through a balloon, they inhaled cannabis vapour on one occasion and a cannabis-placebo vapour on a separate occasion.
Immediately after, they completed a task designed to measure their motivation for earning money. This was a real-life task as the volunteers were given money they had earned at the end of the experiment.
In each trial of the task, volunteers could choose whether to complete low- or high-effort tasks to win varying sums of money.
The low-effort option involved pressing the spacebar key with the little finger of their non-dominant hand 30 times in 7 seconds to win 50p. The high-effort option involved 100 space bar presses in 21 seconds, for rewards ranging from 80p to £2.
"Repeatedly pressing keys with a single finger isn't difficult but it takes a reasonable amount of effort, making it a useful test of motivation," said senior author Professor Val Curran (UCL Clinical Psychopharmacology).
"We found that people on cannabis were significantly less likely to choose the high-effort option. On average, volunteers on placebo chose the high-effort option 50% of the time for a £2 reward, whereas volunteers on cannabis only chose the high-effort option 42% of the time."
In the second study, 20 people addicted to cannabis were matched with 20 control participants who reported the same levels of non-cannabis drug use.
Participants were not allowed to consume alcohol or drugs (other than tobacco or coffee) for 12 hours before the study.
They were then asked to perform the same motivation task as participants in the first study.
The results showed that cannabis-dependent volunteers were no less motivated than the control group. However, the researchers said that a lot more research is needed to fully understand the relationship between long-term cannabis use and potential a motivational deficits.
The research is published in Psychopharmacology.
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