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expert reaction to paper looking at how much skunk there is in the UK cannabis market

A comprehensive survey of cannabis strength – within the UK market – is published in Drug Testing and Analysis.


Prof Valerie Curran, Professor of Psychopharmacology, UCL, said:

“This is an important and timely paper which shows that the UK cannabis street market remains dominated by very high potency varieties containing around 14% THC with negligible cannabidiol (CBD).  This high concentration of THC was also reported in 2005 & 2008, although the present study suggests that this variety is more commonly found now than before.  These findings have implications for the rising numbers of young people who are becoming addicted to cannabis.  Evidence from our own previous research at UCL suggests that high potency varieties are more likely to lead to addiction, so if the market is dominated by these varieties then this inevitably puts more people at risk of addiction.  Marta di Forti’s previous work has suggested that these potent varieties may also increase the individual’s risk of psychosis.”


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

“This is a well-designed and very timely study of cannabis potency in the UK since there has not been one for the last 10 years.  It explores the potency of the most common types of cannabis, resin (or hash), traditional (or herbal) and sinsemilla (or high-potency cannabis) – the latter being the most common and with the highest potency of THC, the main psychoactive compound in cannabis.  The study has found that the overall potency of sinsemilla has not increased over the last decade, but it now makes up a greater proportion of the black market (as measured by police seizures of sinsemilla).  Importantly, the study also found cannabidiol (CBD) a protective and anti-psychotic cannabis compound, to still be lacking in most samples seized by the police.  Resin had roughly 4% THC and 4% CBD a decade ago, this has now changed to roughly 6% THC and 2% CBD – meaning it has become slightly stronger while losing more of its protective CBD.

“This research highlights the need for better monitoring of cannabis potency in the UK, as cannabis potency has been related to various health outcomes.  For example, previous studies have found an association between higher THC levels and greater risk of psychosis or addiction.  A study by Marta Di Forti (King’s College London, KCL), one of the authors of this paper, found that daily users of sinsemilla had a 5-fold increased risk of psychotic illness compared to non-users, while even daily users of resin did not have an increased risk.  It is not yet known whether it is the lower THC content or the higher CBD content which is related to the lower risk of psychosis.  However, a recent clinical trial by the UK drug company GW Pharmaceuticals did find CBD reduced psychotic symptoms in patients with schizophrenia when compared to a placebo.

“Also, a very recent study by Dr Tom Freeman (KCL) looked at THC potency of Dutch cannabis in coffee shops and the number of people seeking treatment for cannabis addiction.  He found that as cannabis potency increased over time, the numbers seeking treatment increased as well, and when potency later decreased the numbers seeking treatment fell too – all with a 5-7-year time lag.  Sometimes it is argued that users will modify their intake accordingly if cannabis potency increases.  However, two carefully conducted studies found that regular users could notice these potency changes and therefore use less – but the changes in use did not fully account for the increases in potency, meaning users still receive a greater amount of THC with the higher potency varieties.

“Furthermore, experimental studies in healthy volunteers who are given THC have found that when CBD is given alongside THC then the memory impairing, paranoia inducing, and addictive effects are reduced.  An ongoing study at KCL is exploring which ratio between THC and CBD are related to the least amount of harm.  However, the relationship between THC/CBD levels in cannabis and cannabis related harms is a very new area of research, and significantly more research is needed to understand these patterns better.”


Prof Michael Lynskey, Professor of Addictions, Institute of Psychiatry, Psychology and Neuroscience, King’s College London, said:

“This paper fills an important gap in our understanding of the cannabis market: despite cannabis being the most commonly used illicit drug in the UK, little is known about types and potency of cannabis being used.  Potter et al document both a remarkable variation in the potency of cannabis products and also a shift in patterns of use over the past decade, with high potency forms of cannabis now dominating the market.  Given the potentially more harmful effects of these products and the emergence of concentrates which may have dramatically higher levels of THC, continued monitoring of the types and potency of cannabis being used is needed.”


Mr Ian Hamilton, Lecturer in Mental Health, University of York, said:

“If these seized samples are representative then it suggests that apart from the dominance of skunk in the UK market, it also seems that resin has increased in strength, as this analysis shows that some resin samples were nearly three times stronger than those seized back in 2005.  So even if people are trying to source lower potency cannabis they are unable to.

“This paper did not look at mental health effects, it just looked at the state of cannabis in the UK.  But we know from previous research that cannabis with higher levels of THC (tetrahydrocannabinol) and lower CBDs (cannabidiols) are associated with risks to mental health, so if the cannabis market is saturated with higher potency cannabis this increases the risk of younger and more naive users developing problems as they are less likely to adjust the amount of cannabis they ingest than more experienced users.”


Prof Celia Morgan, Professor of Psychopharmacology, University of Exeter, said:

“This work is important as much is said about increasing potency of cannabis and declining CBD levels but little is known about actual levels in samples.  Although these are police seizures and hence perhaps not representative of the range of samples used recreationally, the findings are broadly similar to the previous work in 2005.

“This study has found that the strength of sinsemilla or ‘skunk’ on the street has not increased since 2005 and the THC levels are unchanged.  This is in contrast to suggestions that THC concentrations may be rising steadily and that this may be responsible for increasing levels of cannabis dependence.  This is encouraging, suggesting that skunk has not become more dangerous since 2005.  Although it still contains negligible levels of CBD – the potentially protective cannabinoid.

“Unexpectedly, resin or hashish seems to have declining cannabidiol levels and increasing THC levels.  The study also analysed two ultra high potency preparations of butane hash oil for interest, but these were the only instances of these seizures, suggesting that the use of butane hash oil is not prevalent in the UK.  However the number of resin samples was relatively low so perhaps we should not conclude too much in terms of general trends from a relatively small sample.

“Overall, the trend for skunk to dominate the market continues but THC levels in skunk have remained constant over the past decade.”


* ‘Potency of Δ9–tetrahydrocannabinol and other cannabinoids in cannabis in England in 2016: Implications for public health and pharmacology’ by David J. Potter et al. published in Drug Testing and Analysis on Tuesday 27 February 2018. 


Declared interests

None received.



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