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expert reaction to study looking at marijuana use in the USA and perception of harm

The extent of marijuana use in the USA is explored in a paper published in the Lancet Psychiatry journal with the authors reporting an increase in prevalence and frequency of use in adults since 2007 alongside a decreased perception of harm from the drug.


Dr Amir Englund, PhD in Cannabinoid Psychopharmacology, Institute of Psychiatry, Psychology and Neuroscience, King’s College London, said:

“This is a very large and well performed study which provides insights into how many use cannabis in the US and how extensive or problematic their use is. However, it does not allow for any judgements to be made regarding the influence of the changing policy climate in the US with more states both legalising medical and recreational cannabis.

“The authors of the paper report an increase in adult cannabis use while figures indicate a reduction in people presenting with cannabis related problems. This is interesting with regards to the UK as the last decade has seen a slight fall in cannabis use while cannabis related problems have risen by roughly 50% both in adults and adolescents – during a time with little change to policy. One possible explanation for this may be the increase in THC levels of UK cannabis, while the levels of the protective compounds CBD is absent in most preparations. However, the possibility remains that we start seeing more people with cannabis use disorders in the near future.

“Drug use behaviour is not merely a factor of changing policies. This was shown in a recent study which the current study also refers to. It showed a decrease in cannabis use disorder among 12-17 year olds which occurred alongside a reduction in the perceived harms of cannabis. Interestingly, the study showed that the reduction in cannabis use disorder went hand in hand with a reduction in conduct problems (such as stealing and getting into fights). This is further supported by a systematic review article this year (in Lancet Psychiatry, Stocking et al 2016) which found policy to have little or no influence on use or abuse. Psychosocial and socio-political factors play a role as countries with high inequality also show high rates of substance use problems.

“As for the reduction in the perceived risks of cannabis, this needs to be taken into context. It is important to remember that perception of risk is a relative concept – both relative to what the individual previously thought about cannabis but also compared to other activities and drugs (e.g. alcohol and heroin). Some maybe have previously believed classic cannabis scaremongering such as “one toke and you go mad/become a heroin addict”. If such a person changes his/her views, this would be a correct evaluation of the current knowledge. However, it would be wrong to think that cannabis has no risks – so a change from ‘some risk’ to ‘no risk’ would be equally inaccurate in terms of our current understanding.”


Prof. Sir Robin Murray, Professor of Psychiatric Research, Institute of Psychiatry, Psychology and Neuroscience, King’s College London, said:

“From my point of view this is a good study in so far as it goes. It shows that Americans perceive less harm from marijuana as one would expect given the extensive lobbying of the pro-cannabis marketeers. This perception of less harm is not surprisingly accompanied by increased use. However, this kind of study cannot address the big issues i.e. the potential effects of heavy cannabis use on risk of psychosis, road traffic accidents, and cognitive impairments. Different types of study are needed to examine these questions.

Is this a robust study?

“Yes, it is robust concerning consumption and perception of harm.

Can these findings can be applied to the UK?

“No, these findings cannot be applied to the UK – cannabis consumption has gone down over the last ten years in the UK.

Do we know if UK citizens also perceive the drug to be less harmful?

“There are no competent studies of this.

Do we know how significant marijuana use disorders are?

“The authors here looked at self-reported abuse and dependence.  It’s a bit like asking people ‘are you abusing or dependent on alcohol?’ – it’s difficult to know how valid the replies are.

Do these findings cause concern for you?

“Yes – it is concerning that increased use is continuing in the US.”


Prof. David Nutt, The Edmond J Safra Chair and Head of the Centre for Neuropsychopharmacology, Imperial College London, said:

“It’s a pretty robust study as it uses data from a long established and ongoing survey. However they can’t prove causality – it may be the more people use cannabis the more they come to the conclusion it’s less harmful than they thought.”


Dr Michael Bloomfield, Clinical Lecturer in Psychiatry, MRC Clinical Sciences Centre, UCL, said:

“This new study, from an internationally respected group of scientists, seeks to answer the question ‘are changing perceptions around the harms of cannabis associated with changing patterns of cannabis use?’ Using a large and well-conducted survey the authors find evidence in the USA that there has indeed been an increase in cannabis use.

“It is not possible to apply the findings in the UK since one possible factor that may be contributing to the finding is the legal status of ‘medical marijuana’ which does not apply in the UK. This is because it is possible that perceiving the drug as a ‘medicine’ reduces perceptions of harm associated with it, although more research is needed to evaluate that theory.

“This study did not find increased rates of people addicted to cannabis. However, there can be a time-lag between increased use of a drug and increased dependence so this will need careful monitoring.

“This study did not set out to examine cannabis use during adolescence, a time when the mind and brain are still developing, and when it appears most likely that heavy cannabis users may be at highest risk of developing some of the health problems associated with the drug. This will be an important and urgent next step, as increases in heavy use of the drug during adolescence would be concerning for me.

“Likewise it is important to understand how changing patterns of cannabis use related to the use of other drugs. For example, it may be the case that increased use of one drug results in decreased or increased use of other drugs, with associated changes in patterns of harms and benefits.”


‘Marijuana use and use disorders in adults in the USA, 2002–14: analysis of annual cross-sectional surveys’ by Wilson M Compton et al. published in the Lancet Psychiatry on Wednesday 31 August 2016. 


Declared interests

None received.

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