BACKGROUND: In October 2007, the Japanese Health Authority directed that precautions be added to antidepressants (ADs) labelling regarding suicide risk among young people. This study evaluates the impact of the labelling change on AD prescriptions and Japanese adolescent suicide rates. METHODS: We compared AD prescription rates per 100,000 population as a primary outcome. The intervention group comprised adolescents (10-24 years), while the control group comprised adults (25-64 years). We defined the pre-intervention period as January 2005 to October 2007 and post-intervention as November 2007 to February 2013. Monthly prescription rate data from a commercial claims database were triangulated with annual suicide rates in Japan. We performed segmented regression analysis for the prescription rates, using a quasi-Poisson model, and tested for level and trend changes. RESULTS: The commercial claims database included 152,686 adolescents and 195,251 adults during the pre-intervention period and 846,367 adolescents and 1,352,453 adults during post-intervention. Post-intervention, the overall AD prescription rates decreased only in adult males (-95.8 prescription per 100,000) but increased in all other groups. The mean annual suicide rate increased in adolescent males (+1.5 suicides per 100,000) but decreased in all other groups. Overall, the upward trend became moderate or inverse in all groups post-intervention but with a large difference between males and females. The suicide rates rose slightly in adolescents but began declining in adults a year post-intervention. In females, changes in level, trend, and suicide rates were very small in both adolescents and adults. CONCLUSIONS: Contrary to expectations, the mean prescription rates only decreased in adult males, but not in adolescents, regardless of gender. Downward level and trend change were clearly observed in adult males but not in adolescents, the original target of the updated warning. There were no clear temporal associations between suicide rates and the labelling change in either group.