25 years of monitoring the Dutch cannabis market using chemical analys: trends, developments and technical discussion

Background 
Similar to many countries, cannabis is the most-used illicit drug in The Netherlands. Dutch law prohibits the trade, production and possession of cannabis. However, sale for personal use through so-called “coffeeshops” is tolerated under strict conditiions. For 25 years since 1999, the annual “THC monitor” reports the chemical composition of different product types available in coffeeshops, as well as market trends and developments. Following a change in laboratories, the current study also includes an evaluation of the analytical method. 
 
Methods  
For the 2023 edition of the THC monitor, samples were bought in 50 (out of a total of 570) Dutch coffeeshops: most popular (n=70) and (presumably) most potent (n=50) Dutch-grown marihuana, imported hashish (n=61) and marihuana (n=6), and Dutch hashish (n=13). Basic information (price, origin) was also collected. Tetrahydrocannabinol (THC, the main psychoactive compound in cannabis), cannabidiol (CBD) and cannabinol (CBN) were quantified in all samples using GC-FID and HPLC. 
 
Results 
When analyzed using GC-FID, on average, imported hashish contained 19.1%, imported marihuana 0.9%, and Dutch hashish 21.7% THC. The THC content of the most potent and the most popular Dutch marihuana was comparable (12.8% versus 12.6%). The CBD content of imported (mostly Moroccan) hashish has been decreasing in recent years. This might be the consequence of progressive transitioning from traditional hashish-producing methods (i.e. using outdoor-grown, often pollinated plants), towards methods that employ more potent sinsemila plants that are grown indoors (which are not pollinated, leading to even higher cannabinoid concentrations).   
THC levels in Dutch-grown marihuana and imported hashish increased during the second half of the 2010s, yet had been relatively stable for several years until 2022. However, the cannabinoid contents of samples acquired for the THC-monitor are considerably lower than in previous years. Re-analysis of the samples using HPLC – a method more commonly used for cannabis analysis, yet applied in our monitor for the first time – resulted in similar, yet likely more robust results than GC-FID. In the future, cannabis analysis will therefore take place using HPLC. 
 
Conclusions 
The data collected via the “THC monitor” throughout the years allows for careful mapping of the Dutch cannabis market in coffeeshops.  This information can be used to shape prevention and harm reduction policies surrounding cannabis. Similar instruments are also being used in the evaluation of the upcoming “controlled cannabis supply chain experiment”, during which coffeeshops in certain Dutch municipalities will sell legal, quality-controlled cannabis.