PROJECT SUMMARY COVID-19 has led to record high overdose deaths in the US and high risk for future injection-related HIV and hepatitis c (HCV) outbreaks. Innovative approaches are urgently needed to expand access to syringes and naloxone, a drug used for opioid overdose reversal, for the prevention of HIV/HCV outbreaks and opioid overdose fatalities. Expansion and scale-up of mail-based harm reduction services may address harm reduction access gaps by providing a more convenient and confidential way to obtain services. To inform scale-up these services, we aim to 1) describe policy barriers to national expansion of mail-based syringe services and fentanyl strip distribution, 2) conduct a national, longitudinal cohort study to examine predictors of uptake and long-term engagement in mail-based harm reduction services, and 3) assess the add-on harm reduction and health services preferences of mail delivery clients. For Aim 1, we will conduct a systematic legal review of relevant laws in each of the 50 states and the District of Columbia to determine whether and how state law may impact the legality of distributing syringes and fentanyl test strips by mail. We will develop a taxonomy of policies that may prevent legal expansion of mail-delivered syringes. To understand the perception of legal risks associated with mailing syringes among potential adopters, we will survey and interview 20 stakeholders (i.e., health departments and SSPs) from states representing different legal environments for mail-delivery. For Aim 2, we will build on a social media-recruited survey examining uptake and acceptability of mail-delivered harm reduction by expanding the follow up period of that survey from 6 months to 12 and 18 months and expanding the sample size of this survey. This will allow us to assess predictors of uptake and mail-delivered harm reduction engagement over the long-term in order to fully understand who uses mail-delivered services and how these services are being used over time. For Aim 3 we will design and implement discrete choice experiment (DCE) surveys to determine mail-based service client preferences for receiving add-on harm reduction and health services. These results will be disseminated to policy stakeholders, potential adopters, and NEXT affiliates to inform expansion and scale-up of mail-based harm reduction services, improve engagement with current NEXT users, and expand the type of services offered through these programs.