INTRODUCTION: Despite the current opioid crisis resulting in tens of thousands of deaths every year, buprenorphine, a medication that can reduce opioid-related mortality, withdrawal, drug use and craving, is still underprescribed in the emergency department (ED) for treatment of opioid use disorder (OUD). The EMergency department-initiated BuprenorphinE for opioid use Disorder (EMBED) trial introduced a clinical decision support (CDS) tool that improved the proportion of ED physicians prescribing buprenorphine but did not affect patient-level rates of buprenorphine initiation. The present trial aims to build on these findings by optimising CDS use through iterative improvements, refined interventions and clinician feedback to enhance OUD treatment initiation in EDs. METHODS AND ANALYSIS: The Adaptive Decision support for Addiction Treatment (ADAPT) trial employs the Multiphase Optimization Strategy (MOST) framework to refine a multicomponent CDS tool designed to facilitate buprenorphine initiation for OUD in ED settings. Using a pragmatic, learning health system approach in three phases, the trial applies plan-do-study-act cycles for continuous CDS refinement. The CDS will be updated in the preparation phase to reflect new evidence. The optimisation phase will include a 2×2×2 factorial trial, testing the impact of various intervention components, followed by rapid, serial randomised usability testing to reduce user errors and enhance CDS workflow efficiency. In the evaluation phase, the optimised CDS package will be tested in a randomised trial to assess its effectiveness in increasing ED initiation of buprenorphine compared with the original EMBED CDS. ETHICS AND DISSEMINATION: The protocol has received approval from our institution's institutional review board (protocol #2000038624) with a waiver of informed consent for collecting non-identifiable information only. Given the minimal risk involved in implementing established best practices, an independent study monitor will oversee the study instead of a Data Safety Monitoring Board. Findings will be submitted to ClinicalTrials.gov, published in open-access, peer-reviewed journals, presented at national conferences and shared with clinicians at participating sites through email notification. TRIAL REGISTRATION NUMBER: NCT06799117.