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Increasing cannabis potency linked to rising admissions to drug treatment


A new King’s College London study shows that cannabis potency may be linked to rising rates of treatment for cannabis-related problems. Cannabis now accounts for around half of all first-time admissions to specialist drug treatment worldwide - more than for heroin or cocaine - and this new study, published in Psychological Medicine, may offer some explanation for the rise.

Many people use cannabis without any long-term negative effects. However, a minority develop severe problems and may seek specialist treatment to help them quit. Previous laboratory studies have shown that higher doses of THC cause short-term health effects, such as memory impairment and transient psychotic-like symptoms, but long-term health effects of changes in potency are less clear.

‘This study provides the first evidence for a relationship between changes in cannabis potency and healthcare outcomes,’ said lead author Dr Tom Freeman from the Institute of Psychiatry, Psychology & Neuroscience at King’s.

The authors used data from the Netherlands between 2000 and 2015 that showed concentrations of THC – the psychoactive component in cannabis – increased from 9% to 20%, followed by a decrease to 15%. After a five-year delay, the same pattern emerged for first-time cannabis treatment: national admissions per 100,000 inhabitants increased from 12 to 26, before decreasing to 20.

The researchers monitored high-potency, indoor grown herbal cannabis. Referred to as ‘sensimilla’ or ‘skunk’ in the UK, and ‘nederwiet’ in the Netherlands, it is the most common type of cannabis in the UK, USA and Australia.

Monitoring programmes in the Netherlands provide the world’s highest quality data on trends in cannabis potency. Samples are bought directly from ‘coffee shops’, which are officially condoned by the Dutch government. In countries like the UK where all sale of cannabis is strictly prohibited, monitoring programmes have to rely on police seizures, which can be biased due to law enforcement procedures such as stop and search.

The study was observational, so it is possible that factors other than cannabis potency could also explain the upward trend in first-time admissions for drug treatment. For instance, as admissions rose both the proportion of men and the age of individuals seeking treatment decreased. This could be due to changes in referral and treatment practices. Nonetheless, after taking into account alternative explanations, the relationship between potency and admissions remained significant.

The findings may help to explain why treatment for cannabis problems has risen in other countries. In the UK, a study commissioned by the Home Office reported a substantial increase in the market share of high potency forms of cannabis, from 15% in 2002 to 81% in 2008. Following this increase in potency, the number of new people entering treatment for cannabis problems increased by 56% from 2006 to 2014.

The researchers say it is unlikely that an increase in the number of people using cannabis is behind the increase in drug admissions. In the UK, prevalence of cannabis use has actually decreased in recent years.

Around the world, changes in drug laws are having an impact on the kinds of cannabis available. Emerging legal markets in the USA do not include limits or tax on THC, and potent new products are becoming more popular.

‘Our findings highlight a cause for concern regarding the health impact of extremely potent cannabis concentrates which have very recently risen in popularity in some parts of the USA,’ says Dr Freeman. ‘In Washington State, within two years of legalisation over 20% of sales were for “cannabis concentrates” containing an average of 70% THC.’

The study was funded by the Society for the Study of Addiction.


Freeman TP et al (2018). Changes in cannabis potency and first-time admissions to drug treatment: a 16-year study in the Netherlands. Psychological Medicine S0033291717003877

For further media information please contact Robin Bisson, Senior Press Officer, Institute of Psychiatry, Psychology & Neuroscience, King’s College London on or 020 7848 5377.