PROJECT SUMMARY Fentanyl is a synthetic opioid 50-100 times more potent than morphine that is driving a rise in fatal overdoses across the US. Although fentanyl has long been an issue in illicit opioids, in recent years the increased presence of fentanyl in cocaine has drastically increased cocaine-related overdoses. Even though fentanyl poses a major public health threat to cocaine users, there has been no research quantifying how cocaine users respond to fentanyl adulteration. My long-term goal is to use harm-reduction approaches (e.g., overdose education and drug checking technologies) to address fentanyl-related fatal overdoses among people who use cocaine. The objectives of this proposal are: 1. Use behavioral economics to quantify for the first time how cocaine users respond to fentanyl contamination, by examining how contamination affects cocaine demand 2. Examine which individual characteristics may moderate the relationship between fentanyl adulteration and cocaine demand. Thus, I will develop a novel modification of the Cocaine Purchasing Task, which measures cocaine demand using predictive, “real world” metrics of purchase amounts and prices. In my “Adulterated Cocaine Purchasing Task” (ACPT), cocaine users will indicate how much cocaine they would purchase and at what price under low, medium, high and self-judged “typical” probabilities of fentanyl adulteration. To rapidly develop this measure, I will survey N = 200 self-identified cocaine users on Amazon Mechanical Turk, using my novel ACPT and measures of possible moderators including demographics, baseline fentanyl knowledge, etc. My specific aims are: 1. Determine how fentanyl adulteration affects cocaine demand. My working hypothesis is greater probability of fentanyl adulteration will lower cocaine demand. 2. Determine which individual characteristics moderate the relationship between fentanyl adulteration and cocaine demand. My working hypothesis is greater fentanyl knowledge and more opioid experience will relate to reduced demand for fentanyl adulterated cocaine. Expected outcomes are: 1. A novel measure quantifying how fentanyl adulteration in cocaine impacts behavior of cocaine users 2. Identifying individual characteristics that moderate responses to fentanyl adulteration. 3. Necessary training and preliminary data for an F32 or F99/K00 that will test a harm reduction intervention to decrease demand for fentanyl adulterated cocaine. This diversity supplement to the Great Lakes Nodes of the NIDA Clinical Trials Network (UG1-DA049467, funded under the HEAL Initiative) aligns with Aim 1 (new substance misuse research and intervention protocols), Aim 2 (leveraging digital technologies such as MTurk), Aim 4 (support work on substance and opioid misuse), and Aim 5 (professional education on opioid misuse) of the parent grant, and will provide training in behavioral economics, harm reduction approaches, and rapid online data collection to a trainee from an under- represented back...