Today, 80 percent of heroin addicts started out with a prescription for OxyContin. Add to that an equally destructive but less reported epidemic of benzodiazepines (a class of sedatives that includes Valium, Xanax, and Klonopin), and the routine over-prescription of amphetamines like Ritalin and Adderall, and it’s hard not to second-guess our patient-doctor relationships.