Cannabis is not the same as it was 20 years ago. Across Europe and the United States, the potency of cannabis resin and other cannabis products has increased over the past decade. This means that the concentration of tetrahydrocannabinol (THC), the main psychoactive substance in cannabis, has increased in products such as cannabis resin and marijuana.
Denmark is no exception. The average concentration of THC in seized cannabis resin increased from 8.3% in the year 2000 to 31.2% in 2022. In other words, cannabis resin became almost four times as potent over a period of just 23 years. Cannabis resin is also the most commonly used form of cannabis for recreational purposes in Denmark.
This raises an important question: if people are increasingly exposed to stronger cannabis, could this also affect the number of individuals experiencing cannabis-related psychiatric problems? Our new study examined this question at the population level in Denmark.
The increase in potency was associated with three cannabis-related harms
We combined data on THC concentrations in seized cannabis resin with Danish national health and treatment registers. This allowed us to examine whether changes in cannabis potency over time were associated with three indicators of psychiatric harm.
We found positive associations between cannabis potency and all three psychiatric outcomes:
- When cannabis potency was higher, more people subsequently entered specialised treatment for cannabis problems for the first time.
- Higher potency was also associated with more cases of cannabis-induced psychosis, a serious condition that can involve hallucinations and delusions.
- Furthermore, higher potency was associated with more cases of dual diagnosis, defined as being diagnosed with both schizophrenia and a cannabis use disorder.
The associations were strongest when changes in potency and the three psychiatric outcomes occurred around the same time. For cannabis treatment and cannabis-induced psychosis, the associations were also seen several years after changes in cannabis potency.
What does this mean?
Our findings show that cannabis potency is an important factor when considering the health consequences of cannabis use.
While this is consistent with previous research at the individual level, which has linked high-potency cannabis with a greater risk of psychosis and other psychiatric problems, our study adds a population-level perspective: as the potency of cannabis resin increased in Denmark, several indicators of cannabis-related psychiatric harm also increased.
Importantly, our study does not show that stronger cannabis caused the increase in psychiatric problems, as we examined changes across the population rather than following individual cannabis users and measuring exactly how much THC each person consumed.
Over a period of 23 years, many other factors may also change, including patterns of cannabis use, treatment practices, population characteristics, and broader social and health factors. Thus, although our analyses took several such factors into account, an ecological observational study cannot rule out all alternative explanations.
Nevertheless, the findings make an important contribution to existing research linking cannabis use – particularly high-potency cannabis – with psychosis and other mental health problems.
Implications
Our results provide a population-level indication that the increasing potency of cannabis products, such as cannabis resin, may have important public health implications.
The study highlights the need to consider cannabis potency alongside frequency and duration of use in both prevention and treatment efforts. For policymakers, the findings underscore the importance of monitoring changes in the cannabis market and their possible health consequences.
The article is written by Kristine Rømer Thomsen, Birgitte Thylstrup, and Carsten Hjorthøj
Photo: User_76523/Mostphotos
