Cannabis and mental health: why THC potency matters

THC (delta-9-tetrahydrocannabinol) is the molecule in cannabis that produces the high, and the one research associates with mental health disorders. The European EU-GEI study, published in 2019 in The Lancet Psychiatry, compared 901 people seen for a first psychotic episode with 1,237 residents of the same cities, at 11 sites in Europe and Brazil.

Two figures stand out. Compared with never using, daily use multiplied the odds of a psychotic disorder by 3.2, and by 4.8 when that daily cannabis contained 10% THC or more. If the link is causal, the authors estimate that without high-potency cannabis, 30% of first episodes in London and 50% in Amsterdam could have been avoided.

Cannabis sold on French streets is well above that 10% threshold. The French drugs observatory (OFDT), in its report on the drug supply in 2023, gives average THC levels of 29% for seized resin and 14% for herb. Back in 2012, resin averaged 15.9%.

Brain maturation continues past 18, and the age at first use affects the risk. A 2019 meta-analysis in JAMA Psychiatry pooled 11 longitudinal studies of 23,317 people followed from adolescence until they were 18 to 32 years old. Its estimates for young adults who had used cannabis before 18 are:

The authors point out that the individual risk is "moderate to low". The public health problem comes from the number of young people exposed.

Danish registers offer another angle, covering nearly 6.9 million people followed between 1972 and 2021. Drawing on them, a 2023 study linked a cannabis use disorder in boys aged 16 to 20 with a risk of schizophrenia multiplied by 3.84, against 1.81 in girls of the same age. By 2021, 15% of schizophrenia cases in men were attributable to that disorder.

All of these studies are observational: they measure an association, and none can prove on its own that cannabis causes the disorder. Several lines of evidence nonetheless converge:

• the effect increases with dose, frequency and early onset;
• in Denmark, between 1972 and 2021, the share of schizophrenia in men attributable to cannabis grew by an average of 4.8% a year;
• in the EU-GEI study, the cities with the most high-potency cannabis in circulation also have the highest incidence of psychosis.

In its summary of knowledge on cannabis, the OFDT sets out the position of French health authorities: regular use can make some psychiatric disorders worse, and the earlier use starts, the higher the risk of dependence. The same source reports that in 2023, 10.8% of 18-64 year olds and 23.5% of 18-24 year olds had used cannabis in the past year.

CBD (cannabidiol) produces no THC-type high. The Court of Justice of the European Union ruled on it in its Kanavape judgment of 19 November 2020. It noted that, unlike THC, "the CBD at issue does not appear to have any psychotropic effect or any harmful effect on human health", on the current state of scientific knowledge.

There are clinical trials of CBD in mental health. They are small, and their results go in different directions:

Sources: Bergamaschi et al., McGuire et al. and Kwee et al.. Doses ranged from 300 to 1,000 mg of pure CBD, and no authorisation came out of any of these trials. Epidyolex, the only authorised medicine whose sole active ingredient is CBD, is restricted to seizures in three rare conditions; our article on CBD and epilepsy gives the details. What French rules allow a seller to say is covered in our piece on CBD and occasional stress.

A practical precaution follows from this. CBD alters the activity of several liver enzymes that break down antidepressants, anxiolytics and antiepileptics, so if you are under care for a mental health condition, speak to your doctor before taking it. The mechanism is described in our article on CBD and medicine interactions.

What separates street cannabis from a CBD product is THC. The French order of 30 December 2021 caps delta-9-THC at 0.30% in authorised hemp varieties, in their extracts and in the products containing them. The threshold is explained in our article on the legal THC level.

A cap means something only when it is measured. On a certificate of analysis, the Δ⁹-THC line must read below 0.30% and the batch number must match the product. Each line is explained in our guide to reading a certificate of analysis, and the measuring methods on the laboratory page.

Anxiety that takes over daily life, dark thoughts, cannabis use you can no longer control: these are matters for a professional, and no product answers them. Three services are available without a prescription.

• Mon soutien psy gives you up to 12 sessions a year with a partner psychologist at €50 each, 60% reimbursed by the French health insurance. You can book directly without going through your doctor.
• 3114 is the national suicide prevention line: free and open 24 hours a day, 7 days a week.
• Drogues Info Service, on 0 800 23 13 13, is a helpline open 7 days a week where you can talk about your own use or a relative's.

Cannabis use is also handled by youth consultation services and addiction centres (CSAPA). The OFDT counts about 54,000 people followed there for cannabis in 2022.