PURPOSE Brief intervention to reduce cannabis is a promising technique that could be adapted for use in primary care, but it has not been well studied in this setting. We tested the efficacy of a brief intervention conducted by general practitioners among cannabis users aged 15 to 25 years. METHODS We performed a cluster randomized controlled trial with 77 general practitioners in France. The intervention consisted of an interview designed according to the FRAMES (feedback, responsibility, advice, menu, empathy, self-efficacy) model, while the control condition consisted of routine care. RESULTS The general practitioners screened and followed up 261 young cannabis users. After 1 year, there was no significant difference between the intervention and control groups in the median number of joints smoked per month among all users (17.5 vs 17.5; P=.13), but there was a difference in favor of the intervention among nondaily users (3 vs 10; P=.01). After 6 months, the intervention was associated with a more favorable change from baseline in the number of joints smoked (-33.3% vs 0%, P=.01) and, among users younger than age of 18, smoking of fewer joints per month (12.5 vs 20, P=.04). CONCLUSIONS Our findings suggest that a brief intervention conducted by general practitioners with French young cannabis users does not affect use overall. They do, however, strongly support use of brief intervention for younger users and for moderate users.