Cannabis and mental health: why THC potency matters

THC (delta-9-tetrahydrocannabinol) is the cannabis molecule that produces the high. It is also the one research links to 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, across 11 sites in Europe and Brazil.

Two figures sum it up. Daily cannabis use multiplied the odds of a psychotic disorder by 3.2 compared with never using. When the daily cannabis contained 10% THC or more, the factor rose to 4.8. Assuming the link is causal, the authors estimate that 30% of first episodes in London and 50% in Amsterdam could have been avoided without high-potency cannabis.

Street cannabis in France sits well above that 10% threshold. In its report on the drug supply in 2023, the French drugs observatory (OFDT) gives an average of 29% THC for seized resin and 14% for herb. In 2012, resin averaged 15.9%.

The brain keeps maturing after 18, and the age of first use weighs on the risk. A 2019 meta-analysis in JAMA Psychiatry pooled 11 longitudinal studies, covering 23,317 people followed from adolescence to the age of 18 to 32. For young adults who had used cannabis before 18, it calculates:

The authors stress that the individual risk remains "moderate to low". It is the number of young people exposed that makes it a public health problem.

Danish registers add a second angle. Across nearly 6.9 million people followed between 1972 and 2021, a 2023 study found that a cannabis use disorder was associated, in boys aged 16 to 20, with a risk of schizophrenia multiplied by 3.84, against 1.81 in girls of the same age. In 2021, the share of schizophrenia cases attributable to that disorder reached 15% in men.

These studies are observational. They measure an association and cannot, on their own, prove that cannabis causes the disorder. Several lines of evidence point the same way, though:

• the effect grows with dose, frequency and early onset;
• in Denmark, the share of schizophrenia attributable to cannabis rose on average by 4.8% a year in men between 1972 and 2021;
• in the EU-GEI study, the cities where high-potency cannabis circulates most are also those with the highest incidence of psychosis.

The OFDT sums up the position of French health authorities in its summary of knowledge on cannabis: regular use can worsen some psychiatric disorders, and the risk of dependence is higher the earlier use begins. In 2023, according to the same source, 10.8% of 18-64 year olds had used cannabis in the past year, and 23.5% of 18-24 year olds.

CBD (cannabidiol) does not produce the THC high. In its Kanavape judgment of 19 November 2020, the Court of Justice of the European Union noted that, unlike THC, "the CBD at issue does not appear to have any psychotropic effect or any harmful effect on human health", according to the current state of scientific knowledge.

Clinical trials of CBD in mental health do exist, but they are small and their results do not all point the same way:

The sources: Bergamaschi et al., McGuire et al. and Kwee et al.. The doses tested ranged from 300 to 1,000 mg of pure CBD, and none of these trials led to an authorisation. The only authorised medicine with CBD as its sole active ingredient, Epidyolex, is restricted to seizures in three rare conditions, as our article on CBD and epilepsy explains. Our piece on CBD and occasional stress covers what French rules allow a seller to say.

One practical precaution: CBD changes the activity of several liver enzymes that break down antidepressants, anxiolytics and antiepileptics. Anyone under care for a mental health condition should talk to their doctor before taking it. Our article on CBD and medicine interactions describes the mechanism.

The gap between street cannabis and a CBD product is THC. The French order of 30 December 2021 caps delta-9-THC at 0.30% in authorised hemp varieties, their extracts and the products that contain them. Our article on the legal THC level explains the threshold.

That cap only holds if it is measured. On a certificate of analysis, the Δ⁹-THC line must stay below 0.30%, and the batch number must match the product. Our guide to reading a certificate of analysis goes through each line, and the laboratory page describes the measuring methods.

Anxiety that takes over daily life, dark thoughts, cannabis use you no longer control: these call for a professional, not a product. Three services are open without a prescription.

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

Youth consultation services and addiction centres (CSAPA) also take care of cannabis use. According to the OFDT, about 54,000 people were followed there for cannabis in 2022.