Smoking a single spliff of cannabis makes people less likely to work for money, according to new research which is the first to show the short-term effects of the drug on motivation.

In the first of two separate studies, occasional cannabis users were made to inhale the drug's vapour while others were given a placebo. They were both then given the chance to earn different sums of money.

The first task was a low-effort option involving pressing the space bar on a keyboard 30 times in seven seconds to win a prize of 50p. The high-effort option involved 100 space bar presses in 21 seconds for a bigger reward of up to £2.

Senior author Val Curran said 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," the University College London (UCL) researcher said.

The second study suggested that stopping use of cannabis could in fact mean that motivation levels return. Some 20 cannabis addicts, who abstained from the drug for 12 hours, were matched with 20 control participants who previously had not used the drug at all.

They performed the same motivation task as those in the first study, cannabis-dependent volunteers were found to be no less motivated than the control group. The research, which was published in Psychopharmacology, showed as long as users can stop for 12 to 24 hours, they could return to their previous motivation levels.

"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," lead author Dr Will Lawn from UCL said.