The recent US Food and Drug Administration approval of the marijuana constituent cannabidiol as safe and effective for treatment of 2 rare forms of epilepsy has raised hopes that others of the 500 chemicals in marijuana will be found to be therapeutic. However, the long-term consequences of street marijuana use are unclear, and recent studies raise red flags about its effects. Changes in brain maturation and intellectual function, including decreases in intelligence quotient, have been noted in chronic users and appear permanent in early users in most but not all studies.