Significance A widely voiced concern regarding cannabis legalization in the United States is that cannabis is harmful to minors, affecting the developing brain to increase emotional and cognitive problems while impairing functioning. However, these associations may be due to confounding by common vulnerability factors that make some individuals more prone to both cannabis use and negative outcomes. We evaluate this possibility by conducting a series of cotwin control analyses in a sample that combines data from three longitudinal studies. Our findings provide little support for a strong, causal relationship between adolescent cannabis use and adult mental health or cognitive ability but do suggest that cannabis use may impair academic functioning during the secondary school years, with downstream effects on socioeconomic status.