Marijuana is the most frequently used drug in the United States and has been linked to numerous negative outcomes. However, many individuals also perceive marijuana to be beneficial for the treatment of chronic pain. Although evidence for the long-term analgesic effects of marijuana is still preliminary, 15 states have legalized the use of marijuana for medical purposes, including pain relief. To date, even basic data are lacking on the characteristics of those who use medical marijuana and how substance use and substance use-related problems change after obtaining medical marijuana. Similarly, access to medical marijuana may influence an individual's experience of pain, HIV risk profile, functioning, and health service use. With the ongoing policy debate and growing popularity of medical marijuana programs in the United States, it is essential to understand the potential ramifications of medical marijuana use for individuals who seek access to it. The proposed study will screen individuals seeking to obtain medical marijuana at one of four medical marijuana clinics in Southern Michigan. Based on responses to this screen, the proposed project will identify a cohort of 800 individuals (400 men and 400 women) who are seeking medical marijuana for the first time for reasons related to pain. This cohort will be re-assessed at 6-month intervals for 2 years in order to adequately measure the longer-term changes in levels of mental and physical functioning, employment and legal functioning, and health service use outcomes. Initial analyses will utilize the screening data to examine baseline differences in substance use, pain, HIV risk profile, functioning, and service use between those seeking a medical marijuana card for the first time and those seeking to renew their medical marijuana card. Subsequent analyses will model substance use (marijuana and other drug use), pain (pain level and behavioral pain tolerance), HIV risk profile, functioning (mental, physical, employment, and legal functioning), and health service (specialty pain treatment, addictions treatment, and general medical care) utilization over the course of 2 years after a first visit to a medical marijuana clinic for pain treatment. We will also identify factors that predict longer-term trajectories of substance use, pain, functionin, and health service use. As a secondary objective, we will examine the scope of medical marijuana diversion and the characteristics of participants who engage in medical marijuana diversion over time. This project will provide crucial data on individuals seeking medical marijuana for pain management and how these individuals change over time. The resulting data will inform the debate surrounding medical marijuana use and could help shape strategies to identify and intervene with individuals at risk for problems related to substance use.