Primary care is the most frequently utilized health service and is the source of nearly half of all opioids prescribed in the United States. This paper studies the impact of exposure to high prescribing primary care providers (PCP) on opioid use, opioid use disorder, and mental health. Using electronic health records, we exploit within-facility quasi-random assignment of providers, who differ in their opioid prescribing tendency, to 650,000 new patients enrolling in care with the Veterans Health Administration. We find that assignment to a PCP who prescribes opioids at a 2.54 percentage point (pp) higher rate (equivalent to the average difference between a 90th and 10th percentile prescriber within a facility) increases the probability of long-term opioid use by 20% (or 0.43 pp) and development of opioid use disorder by 4% (or 0.035 pp). Veterans’ mental health deteriorates: the three-year likelihood of a depression diagnosis increases by 1.3% (or 0.31 pp). We find that PCPs with more cautious prescribing behavior rely more on non-opioid pain management and adhere more to clinical recommendations on naloxone distribution.