INTRODUCTION: We assessed the effects of the Informed Health Choices (IHC) secondary school intervention on students' ability to think critically about choices 1 year after the intervention. METHODS: We randomized eighty secondary schools to the intervention or control (usual curriculum). The schools were randomly selected from the central region of Uganda and included rural and urban, government, and privately-owned schools. One randomly selected class of year-2 students (ages 14-17) from each school participated in the trial. The intervention included a 2-day teacher training workshop, 10 lessons accessed online by teachers and delivered in classrooms during one school term (May-August 2022). The lessons addressed nine prioritized IHC concepts. We used two multiple-choice questions for each concept to evaluate the students' ability to think critically about choices at both the end of the school term and again after 1 year. The primary outcome was the proportion of students with a passing score (≥ 9 of 18 questions answered correctly) on the "Critical Thinking about Health" (CTH) test. RESULTS: After 1-year, 71% (1749/2477) of the students in the intervention schools and 71% (1684/2376) of the students in the control schools completed the CTH test. In the intervention schools, 53% (934/1749) of students who completed the test had a passing score compared to 33% (557/1684) of students in the control schools (adjusted difference 22%, 95% CI 16-28). CONCLUSIONS: The effect of the IHC secondary school intervention on students' ability to assess health-related claims was largely sustained for at least 1 year. TRIAL REGISTRATION: Pan African Clinical Trial Registry PACTR202204861458660. Registered on 4 April 2022.