The goal of this study is to develop a loading dose approach to starting methadone to treat opioid use disorder with fentanyl use ("fentanyl OUD", herein). This study is a participant- and assessor- blinded dose-finding study using the Bayesian optimal interval (BOIN) design. Investigators aim to recruit n=24 participants with fentanyl OUD to a research unit for monitored methadone initiation. Participants will be randomized to standard initiation vs. loading dose initiation at one of four doses.