A new study investigates the short-term effects of marijuana on human motivation.
The researchers, from University College London (UCL) in the United Kingdom, say that anecdotally, cannabis use has been associated with apathy, lack of motivation, and other reward processing deficiencies.
However, empirical support for these effects is limited, they say.
According to the National Institute on Drug Abuse (NIDA), short-term effects of cannabis use include enhanced sensory perception and euphoria, followed by drowsiness and relaxation.
Other possible effects include: slowed reaction time, balance and coordination problems, increased heart rate and appetite, learning and memory problems, hallucinations, anxiety, panic attacks, psychosis, mental health effects, chronic cough, and respiratory infections.
The researchers of this latest study, led by Will Lawn from UCL Clinical Psychopharmacology, explain that the endocannabinoid system includes the cannabinoid-1 (CB1) and cannabinoid-2 (CB2) receptors.
This system is involved in reward processing and addiction, and the researchers say cannabis dependence has been associated with reduced levels of CB1 receptors.
"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," says Lawn.
To carry out their research, the investigators used 57 volunteers in two different studies.
Cannabis users opted for low-effort task
In the first study, 17 adults who occasionally used marijuana inhaled cannabis vapor on one occasion and cannabis-placebo vapor on another occasion.
Immediately after, the participants completed a task that measured their motivation to earn money. The researchers note that this was a "real-life task," as the participants were given the money they earned at the end of the study.
During each trial, the participants could choose whether to complete low- or high-effort tasks that would earn them varying amounts of money. The low-effort task involved using the little finger of their non-dominant hand to press the spacebar key 30 times in 7 seconds, which earned them 50p (66¢).
Meanwhile, the high-effort task involved pressing the space bar 100 times in 21 seconds to earn between 80p to £2 ($1.06 to $2.65).
Senior author Prof. Val Curran explains that, while continuously pressing keys with one finger is not very difficult, it does take quite a bit 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 percent of the time for a £2 reward, whereas volunteers on cannabis only chose the high-effort option 42 percent of the time."
Prof. Val Curran
Motivation not affected when user is not high
In the second study, 20 participants who were addicted to cannabis were compared with 20 controls who reported the same levels of non-cannabis drug use.
For 12 hours before the study, all participants had to abstain from using alcohol or drugs - other than tobacco or coffee. Then, they were asked to participate in the same motivation task as in the first study.
Interestingly, results showed that the participants who were addicted to cannabis were no less motivated than the participants in the control group, which suggests that long-term cannabis use does not affect motivation when the user is not high.
Though their findings are significant - this is the first fully controlled study to objectively show the acute amotivational effects of cannabis - the researchers note that they could not disregard confounding factors, such as depression.
There were other limitations to the study. For example, the researchers say there were positive drug urine test results for various participants, so residual drug effects could have affected performance.
Additionally, the team did not assess preference of cannabis type in the participants who had cannabis dependence, so they "may have missed out on reward-processing differences between skunk-preferring and hash-preferring participants."
The researchers call for further research to fully understand the link between long-term cannabis use and motivation. Speaking with Medical News Today, Lawn explained that although they do not have any concrete plans as of yet, he and his team would like to expand the study so that it can be completed online.
"That way we would hopefully be able to increase our sample into the hundreds rather than 40," he told us.