Inappropriate prescribing exposes patients to health risks and results in wasteful public expenditures. This study will evaluate an approach to fighting abusive prescription: sending letters to suspected potentially inappropriate prescribers warning them that they are outliers compared to their peers and have been flagged for review. The study will target high prescribers of opioids in the Schedule II controlled substances class. Two types of letters will be tested: one focusing on the health consequences of inappropriate prescribing for patients, and the other focusing on the consequences for prescribers including e.g. potential administrative actions. Using claims data, the investigators will assess the effect of the letters on prescribing of opioids, receipt of opioids by patients, substitution behavior by prescribers and patients, and health outcomes of patients.