Veterans who have experienced repeated blast exposures can develop a range of long-term consequences of mild traumatic brain injuries (mTBI). Their injuries also increase the likelihood of prolonged opioid use due to pain related to the injuries and resulting treatment with opioids. The long-term consequences of mTBI include an increased risk for developing PTSD and substance use disorders (SUDs), and this risk is likely compounded by prolonged opioid exposure. Individual genetic and life experience factors can also contribute to the risk of developing PTSD and SUDs. For example, the FKBP5 gene has sequence variants in the population that alter the risk for developing PTSD and SUDs. It encodes the protein FKBP51, which regulates glucocorticoid receptor sensitivity. This study will investigate the additive effects of mTBI and prolonged opioid exposure while asking whether inhibiting FKBP51 can ameliorate the long-term risks of mTBI and opioids. We propose to use male and female mice in a fully parametric design to test whether mTBI and prolonged opioid exposure exacerbate the risk for anxiety-like and drug seeking behaviors later and to test a possible therapeutic solution. Mice will be exposed to overpressure blast in a well-defined protocol that is routinely used at our facility followed by prolonged (four week) and continuous treatment with fentanyl. Four weeks later, we will test the animals for the development of anxiety behaviors and their vulnerability to take fentanyl using a novel oral self-administration procedure. We predict that animals that received both blast and prolonged opioids will have worsened anxiety-like and fentanyl seeking behaviors afterwards. We will then test whether SAFit2, a potent and selective FKBP51 small molecule antagonist, can mitigate these outcomes. Our goal will be to translate these preliminary experiments into a Merit Award to test these ideas more fully in hopes of developing an innovative new treatment for these long term stress-associated and substance use problems.