BACKGROUND & OBJECTIVEMobile substance use treatment units are effective approaches to increase treatment access and reduce barriers to opioid use disorder (OUD) care. However, little is known about the economic costs of maintaining and operating these units. This study aimed to estimate the economic costs of starting and maintaining mobile units providing harm reduction, overdose education and naloxone distribution (OEND), and medication for opioid use disorder (MOUD).METHODSAs part of the HEALing Communities Study, four communities in Massachusetts (Bourne/Sandwich, Brockton, Gloucester, Salem) implemented mobile units offering OEND and MOUD (buprenorphine and naltrexone only); each selected different services tailored to their community. All provided MOUD linkage via telehealth, but only one offered in-person MOUD prescribing on the unit. We retrospectively collected detailed resource utilization data from invoices to estimate the direct economic costs from August 2020 through June 2022. Cost components were categorized into start-up and operating costs. We calculated total economic cost over the study period and the average monthly operating cost.RESULTSImplementing a mobile unit offering OEND and MOUD required a one-time median start-up cost of $59,762 (range: $52,062-$113,671), with 80 % of those costs attributed to the vehicle purchase. The median monthly operating cost was $14,464. The largest cost category for all mobile units was personnel costs. The monthly ongoing costs varied by community settings and services: approximately $5000 for two urban communities offering OEND and MOUD linkage via telehealth (Gloucester, Salem), $28,000 for a rural community (Bourne/Sandwich), and $23,000 for an urban community also providing in-person MOUD prescribing on the unit (Brockton).CONCLUSIONThe economic costs of mobile substance use treatment and harm reduction units are substantial but vary by community settings and services offered. Our results provide valuable community-level economic data to stakeholders and policymakers considering establishing and/or expanding mobile units with OEND and MOUD services. Further exploration of cost-effectiveness and efficiency should be considered across different settings.