If you smoke marijuana, you may be setting yourself up to be a loser. A British study found that smoking a single joint made people less willing to work for money. It is the first study to demonstrate the short-term effects of marijuana in humans.

Occasional cannabis users inhaled the equivalent of one cigarette through a balloon. They were then asked to choose between two computer tasks involving pressing the space bar with the little finger of their non-dominant hand for up to 21 seconds.

The test was designed to measure their motivation to work for money — an option that required little effort for a minimal amount of money, and a second that required more effort for up to four times the money.

The same group repeated the test at a later time, but inhaled a vapor that contained no marijuana, and the results of both sessions were compared.

"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," explained senior author Val Curran of University College London (UCL).

"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 percent of the time for a £2 reward, whereas volunteers on cannabis only chose the high-effort option 42 percent of the time."

The results may be due to levels of dopamine, the "feel good" chemical produced in the brain that's also linked to motivation. A earlier study at UCL found that levels of dopamine are lower in heavy users and those who began the drug at a younger age.

"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," said lead author Dr. Will Lawn.

The study was published in Psychopharmacology.

Marijuana use continues to rise in the United States. A recent survey found that the percentage of adult users rose from 21.9 million to 31.9 million between 2002 and 2014.