Smoking cannabis ‘dampens motivation in the short term’

    6 September 2016

    Smoking just one cannabis cigarette makes people less willing to work for money while under the influence of the drug, according to a study published in the journal Psychopharmacology. The study found that long-term users experience no loss of motivation.

    The researchers, from University College London, say this is the first study to reliably demonstrate the short-term effects of cannabis on motivation. However, the paper has been criticised (see our expert analysis below).

    Long-term cannabis addicts (who were not high at the time) were also tested, and it was found that their motivation levels were no different to volunteers in the control group.

    During the study 57 volunteers were asked to complete a task, which, if performed successfully, would earn them real money. Prior to the task, participants inhaled either cannabis vapour or cannabis-placebo vapour.

    In each trial, volunteers could choose between low- or high-effort tasks to win varying sums of money. The low-effort option involved pressing the spacebar key with the little finger of their non-dominant hand 30 times in seven seconds to win 50p. The high-effort option involved 100 space bar presses in 21 seconds, for rewards varying from 80p to £2.

    Senior author Professor Val Curran said: ‘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. 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 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.’

    The study’s lead author, Dr Will Lawn, 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.

    ‘It has also been proposed that long-term cannabis users might also have problems with motivation even when they are not high. However, we compared people dependent on cannabis to similar controls, when neither group was intoxicated, and did not find a difference in motivation.

    ‘This tentatively suggests that long-term cannabis use may not result in residual motivation problems when people stop using it. However, longitudinal research is needed to provide more conclusive evidence.’

    Instant analysis
    Two compounds were studied: THC (tetrahydrocannabinol) and CBD (cannabidiol). Each of these compounds exists in different concentrations in different preparations of cannabis and it is suggested that for chronic users, the tendency is for a choice of high-THC, low-CBD preparation. It is postulated that CBD buffers against some of the negative effects of cannabis usage — apathy, memory loss, anxiety and psychosis.

    The team sought to establish the effects of these drugs on motivation, as prior work reached conflicting conclusions. The first experiments looked at the effect of cannabis without CBD and with CBD. The study had two segments: lower-frequency cannabis users, and then subsequently cannabis-dependent users.

    It is important to note is that the subjects did not always win the money after completing the tasks. The probability of winning the money was set either at 11 per cent, 50 per cent and 88 per cent.

    The study showed that, as expected, cannabis (with CBD and without) reduced the likelihood for making high-effort choices and therefore lowered motivation in the short term. It also suggested that long-term consumption did not produce the same ‘amotivational’ response.

    However, the study was small – 20 and 17 subjects respectively – and packed with confounders.

    For instance, most of the subjects tested positive not just for THC but for PCP, opiates, MDMA, cocaine and benzodiazepines. A strong argument is that the reward in the study was greater than the scope of the actual tasks. The drug users may have been earning quick money for their habit and therefore have been exceptionally motivated.

    Consequently, this study merely provides more conflicting data to an already conflicted area.

    I confess to a certain degree of apathy about the work itself. The research subjects would be difficult to study and unreliable. What truly is the end goal?

    What I have experienced in practice confirms my suspicions already. Often I have seen people dragged down to the surgery by distraught mothers asking if I can ‘give them something’ to make them want to do things. Often my suggestion instead is to take ‘something’ away, which is a lot more difficult to achieve.
    Research score: 1/5