Changes in cannabis potency and first-time admissions to drug treatment: a 16-year study in the Netherlands
Tom P. Freeman, Peggy van der Pol, Wil Kuijpers, Jeroen Wisselink, Ravi K. Das, Sander Rigter, Margriet van Laar, Paul Griffiths, Wendy Swift, Raymond Niesink and Michael T. Lynskey
Psychological Medicine, 2018
Background : The number of people entering specialist drug treatment for cannabis problems has increased considerably in recent years. The reasons for this are unclear, but rising cannabis potency could be a contributing factor.
Methods : Cannabis potency data were obtained from an ongoing monitoring programme in the Netherlands. We analysed concentrations of δ-9-tetrahydrocannabinol (THC) from the most popular variety of domestic herbal cannabis sold in each retail outlet (2000–2015). Mixed effects linear regression models examined time-dependent associations between THC and first time cannabis admissions to specialist drug treatment. Candidate time lags were 0–10 years, based on normative European drug treatment data.
Results : THC increased from a mean (95% CI) of 8.62 (7.97–9.27) to 20.38 (19.09–21.67) from 2000 to 2004 and then decreased to 15.31 (14.24–16.38) in 2015. First-time cannabis admissions (per 100 000 inhabitants) rose from 7.08 to 26.36 from 2000 to 2010, and then decreased to 19.82 in 2015. THC was positively associated with treatment entry at lags of 0–9 years, with the strongest association at 5 years, b = 0.370 (0.317–0.424), p < 0.0001. After adjusting for age, sex and non-cannabis drug treatment admissions, these positive associations were attenuated but remained statistically significant at lags of 5–7 years and were again strongest at 5 years, b = 0.082 (0.052–0.111), p < 0.0001.
Conclusions : In this 16-year observational study, we found positive time-dependent associations between changes in cannabis potency and first-time cannabis admissions to drug treatment. These associations are biologically plausible, but their strength after adjustment suggests that other factors are also important.
Cannabis is used by an estimated 183 million people, and accounts for around half of all firsttime admissions to specialist drug treatment worldwide (UNODC, 2016). The number of people entering specialist drug treatment for cannabis problems has risen considerably in recent years. Across Europe, there was a 53% increase in first-time clients between 2006 and 2014, and cannabis has now superseded opiates as the primary problem drug (EMCDDA, 2016). These changes highlight a concerning increase in population markers of burden and morbidity attributable to cannabis. There are no approved pharmacotherapies for the treatment of cannabis use disorders, and psychosocial interventions have limited efficacy (Curran et al. 2016). The increase in cannabis admissions, alongside a lack of evidence based interventions creates a significant challenge for treatment providers (Monaghan et al. 2016). Clients entering specialist drug treatment with cannabis as a primary problem have shown the poorest treatment outcomes at 6 months (rates of abstinence and reduction in use) of all illicit drugs (NDTMS, 2014).
Use of cannabis products with high concentrations of THC has been linked to poorer mental health and addiction outcomes (Di Forti et al. 2015; Freeman & Winstock, 2015; Schoeler et al. 2016; Chan et al. 2017; Meier, 2017). A cross sectional online survey (Freeman & Winstock, 2015) found that use of cannabis with high THC content was more strongly associated with cannabis dependence than lower potency forms of cannabis. Moreover, this association was found to be stronger in younger cannabis users. A second cross-sectional online survey found that use of extremely high potency cannabis concentrates (Butane Hash Oil) was associated with greater physical dependence on cannabis, and this association was robust after accounting for possible confounds using both covariate adjustment and propensity score matching (Meier, 2017). Prospective studies have reported an association between degree of cannabis exposure and transition to cannabis dependence (Silins et al. 2014), although not in those who are using cannabis (near) daily at baseline (van der Pol et al. 2013a). Naturalistic studies suggest that cannabis users only partially adapt their smoking behaviour to differences in cannabis potency (Freeman et al. 2014; van der Pol et al. 2014). Taken together, it is plausible that long-term changes in cannabis potency could influence cannabis-related harms (including changes in cannabis admissions to drug treatment). Although the potential health impacts of increasing cannabis potency have been widely acknowledged (McLaren et al. 2008; Di Forti et al. 2015; Freeman & Winstock, 2015; ElSohly et al. 2016; EMCDDA, 2016; UNODC, 2016) we are unaware of any previous attempts to empirically test associations between changes in cannabis potency and population markers of cannabis harms.
Effective monitoring of cannabis potency can play a critical role in estimating the potential health impact of cannabis use in different regions. However, high-quality and long-term monitoring programmes are extremely rare (Freeman & Swift, 2016). Of those available, the Trimbos Institute potency monitor (Pijlman et al. 2005; Niesink et al. 2015) offers the highest quality evidence and is the most suitable resource for testing associations between changes in potency and cannabis harms. Firstly, cannabis samples are purchased directly at the retail level from ‘coffee shops’ using randomised sampling. This method is advantageous to other studies utilising cannabis samples from police seizures, which may be biased by law enforcement methods (Nguyen & Reuter, 2012), sampling bias and variation in sample degradation during storage (Sevigny, 2013). Secondly, in contrast to linear increases in cannabis potency reported elsewhere (ElSohly et al. 2016), THC concentrations have both risen (Pijlman et al. 2005) and then subsequently declined (Niesink et al. 2015) in the Netherlands during the last 16 years, providing a unique opportunity to detect similar changes in cannabis-related problems (Freeman & Swift, 2016). Here we sought to test whether changes in cannabis potency (THC) are associated with rates of first-time cannabis admissions to specialist drug treatment in the Netherlands from 2000 to 2015. This study was reported according to the STROBE (strengthening the reporting of observational studies in epidemiology) statement.