We study a regulation in Chile that mandates front-of-package warning labels on products whose sugar or caloric concentration exceeds certain thresholds. We document an overall decrease in sugar and caloric intake of 7-9%. To unpack the underlying mechanisms, we provide descriptive evidence of the impact of the policy on consumer choice, both across and within categories and firms’ behavior. We  find no noticeable substitution of products across food categories and show that most of the demand effect of the regulation comes from within-category substitution. We also find that a substantive portion of the overall effect comes from product o reformulation. We discuss how these  findings can inform the design of effective labeling policies.