AIMS: To test the effectiveness of a digital intervention to reduce cannabis use (ICan) with adherence-focused guidance compared with educational cannabis information.

DESIGN: This was a single-blind randomized controlled trial. Follow-up clinical outcome measurements took place 3 and 6 months after randomization. The trial was conducted in the Netherlands. The intervention and guidance took place on-line, with recruitment via Facebook/Instagram advertisement campaigns. Inclusion criteria were ≥ 18 years, cannabis use on ≥ 3 days/week, the desire to reduce/quit cannabis and using a smartphone. Participants were allocated to either ICan (n = 188) or control (n = 190) (69% male, mean age = 27.5 years).

INTERVENTION AND COMPARATOR: ICan is a mobile (web-)application based on motivational interviewing and cognitive behavioural therapy and includes three main components: screening, brief intervention (six modules) and referral to treatment. The control condition consisted of non-interactive educational cannabis information.

MEASUREMENTS: Primary outcome was the number of cannabis use days in the 7 days prior to the 6-month follow-up measurement. Secondary outcome measures at 3- and 6-month follow-up were the number of grams of cannabis used and attitudes towards seeking professional help for cannabis use related problems.

FINDINGS: Intention-to-treat analysis showed that 6 months after randomization the mean number of cannabis use days in the past 7 days was reduced in both conditions (time P < 0.001), with no significant group × time interaction effect. Three months after randomization the mean number of grams used in the past 7 days was reduced in both conditions, with a significantly larger reduction in the ICan condition. At 6-month follow-up the significant group × time interaction effect was no longer present (P = 0.30). In both conditions, attitudes towards seeking professional help remained virtually unchanged over time.

CONCLUSIONS: A digital intervention to reduce cannabis use (ICan) was more effective than non-interactive educational cannabis information in reducing grams of cannabis used over 3 months, but not more effective at reducing cannabis use days at 6-month follow-up. Cannabis use reductions were maintained in both conditions between 3 and 6 months' follow-up.