Introduction: A 2008 survey found that 1.9% of the entire US population was using prescription pain medication non-medically and that 56% obtained them from a friend or relatives. Diversion of pain medication may occur when a portion of the prescription is unused for pain relief after an ED visit. We hypothesized that at least 10 pills (~40%) of an opioid prescription 2 weeks after an ED visit, will not be consumed and become available for potential misuse. Objective: Determine the quantity of unused opioids pills for common acute pain diagnoses, 2 weeks after an ED visit for acute pain. Methods: Prospective observational cohort study of consecutive ED patients from a tertiary academic urban hospital with 60,000 ED visits annually. Inclusion criteria: aged ≥18 years, acute pain conditions present ≤2 weeks, pain intensity at triage of ≥4 (on a 0-10 numeric rating scale; NRS), and discharged with a new opioid prescription. ED physicians identified (24/7) eligible patients. They recorded the pain complaint/location, the final diagnosis, the quantity and type of prescribed pain medication. Discharged patients completed paper or electronic 14-day diary (REDCap database) to document their pain medication consumption. As a mitigation strategy, they were also contacted by phone at 2 weeks for the same information. A paired t-test was used to test the difference between the amounts of opioids prescribed and consumed. Results: 350 patients were recruited. Mean age 50 (SD ±16) and 54.2% were men. Painful diagnosis: fracture (18.2%), acute back pain (15.3%), renal colic (15.3%), Sprain (excluding back/neck pain) (6.9%), Contusion (6.4%), acute neck pain (5.8%), abdominal pain (4.9%), and other (27.2%). Opioids prescribed: oxycodone (47%), morphine (37%) and hydromorphone (16%). Means quantity of opioid pills prescribed: 24 (IC95%: 23-26). Filled opioid prescription: 92%. Means quantity of opioid pills consumed: 8 (IC95%: 7-9). Means quantity of unused opioids pills: 16. Opioid pills available for misuse in our cohort: 5,600 pills. Conclusion: After an ED visit for acute pain a significant portion of opioids prescribed is unused and available for misuse. A large pragmatic study should be done to confirm that an opioid prescription strategy based on our results will limit unused opioid pills while maintaining pain relief.