Every day, Dr. Carl Hart goes into his laboratory at Columbia University and gets people high. That research has led him to a surprising conclusion: the predominant effects of the drugs he administers — substances like cannabis, cocaine, heroin, and meth — are positive. In this unflinching conversation about Dr. Hart’s bold new book, “Drug Use for Grown-Ups,” Carl and Rufus discuss their own experiences with drugs, connect drug criminalization to structural racism, and ask whether drug use by responsible adults can be a good thing.