However the researchers found no difference between cannabis users not under the influence of the drug and non-users – at least when it came to a test that involved tapping their little finger really quickly.
The study, detailed in a paper in the journal Psychopharmacology, saw 17 occasional cannabis users given a vapour containing the drug and also a vapour containing a placebo.
They were then given a choice of carrying out a relatively easy task – tapping the spacebar on a computer keyboard 30 times in seven seconds – or a hard task – the same thing, but 100 taps in 21 seconds.
The easy task paid out 50p, while the hard one resulted in a payment of between 80p and £2.
One of the researchers, Professor Val Curran, of University College London, said: “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.
“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 per cent of the time for a £2 reward, whereas volunteers on cannabis only chose the high-effort option 42 per cent of the time.”
In a second test, the researchers compared 20 “cannabis-dependent individuals” with 20 control people. No one was allowed to consume alcohol or drugs – apart from cigarettes or coffee – for 12 hours before the study.
However this time the two groups showed no differences in their willingness to tap the spacebar.
Dr Will Lawn, another of the UCL researchers, admitted cannabis was “commonly thought to reduce motivation”.
But he added: “This is the first time it has been reliably tested and quantified using an appropriate sample size and methodology.
“It has also been proposed that long-term cannabis users might also have problems with motivation even when they are not high.
“However, we compared people dependent on cannabis to similar controls, when neither group was intoxicated, and did not find a difference in motivation.
“This tentatively suggests that long-term cannabis use may not result in residual motivation problems when people stop using it. However, longitudinal research is needed to provide more conclusive evidence."
By Ian Johnston