Most pot smokers experience hallucinations, dizziness, and an increase in appetite after a few hits. The physical and psychological effects of THC (the main active ingredient in marijuana) wear off after an hour or two, but researchers have found it can have a short-term effect on motivation. A single "spliff" of pot blunts smokers' motivation to work for money while high, according to a recent study published in Psychopharmacology.

"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 Dr. Will Lawn (UCL Clinical Psychopharmacology), lead author of the study, in a statement.

Previous research has found excessive pot use stifles motivation via the decrease of dopamine levels in the striatum, a brain region that functions as part of the reward system. Dopamine acts as a "motivation molecule" by enhancing concentration, boosting mood, and has a pro-social effect. Lower dopamine levels have been linked to users who meet diagnostic criteria for cannabis abuse or dependence.

Lawn and his colleagues found the short-term effects of pot on motivation are significant in the two-part study. First, 17 adult volunteers who used marijuana occasionally inhaled the cannabis vapor through a balloon on one occasion and a placebo vapor on a separate occasion. Immediately after, the participants were asked to complete a task designed to measure their motivation for earning money.

Woman smoking pot Smoking pot may blunt motivation in users. Photo courtesy of ashton, Cc By 2.0

Volunteers could choose whether to complete low- or high-effort tasks to win various sums of money. In the low-effort option, smokers had to press the spacebar key with the little finger of their non-dominant hand 30 times in 7 seconds to win 50 pence. The high-effort option involved 100 space bar presses in 21 seconds, for rewards varying from 80p to £2.

The researchers found people were significantly less likely to choose the high-effort option when they were high compared to the placebo group. The latter chose the high-effort option 50 percent of the time for a £2 reward, whereas the high group chose the high-effort option 42 percent of the time.

“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 Professor Val Curran (UCL Clinical Psychopharmacology), senior author of the study, in a statement.

In the second experiment, 20 people addicted to pot were matched with 20 control participants who reported the same levels of non-cannabis drug use. They were asked to perform the same motivation task as the prior group, but were not allowed to consume alcohol or drugs, other than tobacco or coffee, for 12 hours before the study. Interestingly, those who were addicted to pot, but not high during the test, showed the same motivation levels as the control group.

This tentatively implies the effects of pot on dopamine levels are likely to be reversible when users stop. When people stop using pot, the brain may slowly go back to producing normal levels of dopamine, and therefore, increase motivation. A 2012 study suggests frequent pot smokers do not suffer from lasting changes in dopamine levels, unlike users of other common drugs.

Researchers are still seeking to better understand the relationship between long-term pot use and its effects on motivation, or lack thereof.

Source: Lawn W and Curran V. Cannabis reduces short-term motivation to work for money. Psychopharmacology. 2016.