The study raises fears that long-term use of the drug could affect an individual’s motivation to work even when they are not high on cannabis

Smoking just one cannabis joint reduces a person’s motivation to work for money, research suggests.

For the first time, scientists have confirmed that cannabis users who inhale a small amount are less likely to choose a high-effort task while intoxicated.

It will raise fears that long-term use of the drug could affect an individual’s motivation even when they are not high – and therefore their life chances – although researchers did not confirm this.

Study leader Dr Will Lawn, from University College London, said: ‘Although cannabis is commonly thought to reduce motivation, this is the first time it has been reliably tested and quantified using an appropriate sample size and methodology.’

The research, published in the Psychopharmacology Journal, involved 57 volunteers split over two separate studies.

The first saw 17 occasional cannabis users inhale the equivalent of a single joint of cannabis through a balloon and were offered the choice of two tasks to earn different sums of money.

The low-effort option involved pressing the spacebar key of a computer with their little finger 30 times in 7 seconds to win 50p.

The high-effort option involved 100 space bar presses in 21 seconds, for rewards varying from 80p to £2.

The same group of people repeated the test on a separate occasion, but inhaled ‘placebo’ vapour which contained no cannabis.

Researcher Professor Val Curran said: ‘We found that people on cannabis were significantly less likely to choose the high-effort option.

The study found smoking just one cannabis joint reduced a person’s motivation to work

‘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 the second study, 20 people addicted to cannabis were matched with 20 people who never smoked cannabis.

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 suggested that cannabis-dependent volunteers were no less motivated than the control group – but the authors stressed that much more research is needed before any conclusions can be reached.

Lucy Dawe, spokesman for charity Cannabis Skunk Sense, said: ‘We deal quite often with youngsters who are high achievers before they start smoking cannabis, but see their grades drop considerably as a result.’