Abstract: Medication treatments for opioid use disorder (MOUD) save lives and improve outcomes for countless individuals. However, data suggest the potential for significant weight gain during methadone treatment and little is known about weight change during buprenorphine treatment. Using Veteran Health Administration administrative data from fiscal year 2017 to fiscal year 2019, two cohorts were created: 1) Veterans diagnosed with opioid use disorder (OUD) taking methadone (N = 1425); and 2) Veterans diagnosed with OUD taking buprenorphine (N = 3756). Linear mixed models were used to analyze weight change during the first MOUD treatment episode in the observation period. Random slopes and intercepts were included in the model to estimate variation in BMI across individuals and time. The data revealed a slight upward trend in BMI over the course of treatment. Specifically, a daily increase of 0.004 for Veterans in methadone treatment and 0.002 for Veterans in buprenorphine treatment was observed. This translates to a gain of about 10 pounds over the course of 1 year of methadone treatment and 5 pounds for 1 year of buprenorphine treatment for a Veteran of average height and weight. The amount of weight gain in methadone treatment is significantly less than other published Findings, but nonetheless indicates that assessment and discussions between patients and providers related to weight may be warranted.