Opioids are an effective, widely prescribed, and highly addictive class of drugs used for pain management. The consequent broad access to these drugs has contributed to the opioid epidemic, and drug overdoses are now the leading cause of death for Americans under 50. Using data on hundreds of millions of medical claims and 19 million opioid prescriptions in Massachusetts between 2010 and 2015—by then the 7th highest state for death rate by opioid-related overdoses—I will introduce evidence that the diffusion of these medications within families has contributed to the proliferation of opioid consumption. Moreover, these effects are unlikely to be explained away by shared traits, environment, or mate selection. This empirical perspective on the opioid epidemic within families—a ubiquitous social structure—has important implications for policy.