*2.1. Study Population and Inclusion Criteria*

Using a multi-layered strategy, IPSOS Health, an independent market research institution, recruited randomly selected potentially eligible physicians via its panels and business directories (e.g., directory of the National Association of Statutory Health Insurance Physicians). The first contact was made by e-mail. Interested physicians were then contacted by phone. The final intervention was online. All participants were reimbursed for participation by IPSOS Health. To detect a 15% difference in the randomized ERONA

trial, 300 participants per intervention arm were recruited between April 2020 and August 2020, resulting in a sample of 600 physicians over a wide range of disciplines. Details of the sample size calculation were already published [25]. Only physicians prescribing strong opioids regularly for CNCP were included in the study and detected by the screener question: "Do you prescribe BtM (Betäubungsmittel)-based opioids to treat patients with chronic, non-tumor-related pain?". For strong opioids in Germany, it is necessary to make the prescription on a special prescription form, called *BtM*-prescription. Informed consent was obtained online prior to the study.
