Warfighters in far-forward deployed settings are at risk for exposure to weaponized opioids and subsequent opioid intoxication. Naloxone is ordinarily effective for immediate opioid reversal. However, synthetically derived opioids such as carfentanil and fentanyl-derivatives have a half-life that exceeds naloxones, subjecting victims to a phenomenon known as renarcotization, requiring repeated doses of naloxone over many hours. The DoD seeks to develop a long-duration medical countermeasure (MCM) to counter this phenomenon. Such an MCM is paramount for soldiers to remain ambulatory to complete the mission or move though a clean/dirty line for transition to a higher echelon of medical care. This Phase I SBIR project will reformulate naloxone using FDA-approved microencapsulation technology into a long acting injectable that can provide 12-24hrs of sustained antagonist activity, combined with free naloxone for immediate overdose reversal. The formulation will be suitable for autoinjector use, and approval can be sought through the FDA 505(b)(2) accelerated regulatory pathway providing a near-term entry into the market. Successful completion of this project will provide a stable, intramuscular formulated drug product for a long duration opioid antagonist MCM to provide both immediate relief from opioid poisoning as well as prolonged protection from renarcotization for Warfighters exposed to synthetic opioids.