BACKGROUND AND AIMS: Lower risk guidelines for safer levels of cannabis use could help to reduce the health burden posed by cannabis use disorder (CUD). We aimed to estimate risk thresholds for CUD based on delta-9-tetrahydrocannabinol (THC) consumption using standard THC units (1 unit = 5 mg THC).

DESIGN: Data from the CannTeen study, a longitudinal observational study consisting of five assessments over a 12-month period in London, UK. Participants were n = 65 adults aged 26-29 (46% female, 70.77% white ethnicity) and n = 85 adolescents aged 16-17 (56% female, 65.48% white ethnicity). All participants reported at least one use of cannabis during the 12-month study period.

MEASUREMENTS: Mean weekly standard THC units were estimated using the Enhanced Cannabis Timeline Followback, a comprehensive and validated assessment of quantity, frequency and potency of cannabis consumed. This was administered at 3-month intervals and averaged over a 12-month period. Past 12-month diagnosis of CUD using The Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5) was assessed at the final follow-up. Receiver operating characteristic curve models estimated the extent to which weekly standard THC unit consumption could discriminate no CUD from any CUD (mild, moderate or severe), and no CUD from moderate/severe CUD, in adults and adolescents separately. Risk thresholds were selected based on cut-offs that maximised sensitivity and specificity.

FINDINGS: Discrimination accuracy of weekly standard THC units on CUD was good, with area under the curve > 0.70 for all models. Optimal cut-offs for risk of any CUD (versus no CUD) were 8.26 units per week for adults and 6.04 units per week for adolescents. For risk of moderate/severe CUD (versus no CUD) optimal cut-offs were 13.44 units per week for adults and 6.45 units per week for adolescents.

CONCLUSIONS: Weekly cannabis consumption based on standard delta-9-tetrahydrocannabinol (THC) units appears to show good discrimination accuracy of cannabis use disorder at different severities and in different age groups. Safer levels of cannabis use, defined by low weekly standard THC unit consumption, could be recommended in lower risk cannabis use guidelines.