Abstract: Opioid analgesics are commonly used to manage children’s postoperative pain. Under-medication of children’s postsurgical pain can have significant consequences including reduced quality of life, respiratory issues, dehydration, and nausea, which can increase health care costs due to emergency department visits, provider phone calls, and readmissions. In contrast, a proportion of parents may over-medicate children’s postsurgical pain, potentially leading to greater respiratory depression and other opioid side effects. Given (a) the potential repercussions of under- or over-medicating postsurgical pain in children, (b) uncertainty regarding level of opioid analgesics needed for optimal postsurgical pain management, and (c) increasing societal focus on the negative consequences of opioid medications, further understanding the patterns and predictors of home opioid use and pain management following surgical procedures in children is necessary to guide development of interventions to enhance appropriate analgesic use for pediatric postsurgical pain. The proposed project is designed to address this gap in understanding by prospectively examining parent and child predictors of the extent of opioid analgesic use following tonsillectomy and adenoidectomy (T&A) procedures in pediatric patients aged 7 to 12 years. Parents and youth will complete preoperative measures regarding opioid medication beliefs, pain-related beliefs and emotional responses, and history of painful experiences and opioid use. At the time of IV placement for surgery, blood will be collected to assess children’s circulating endocannabinoid levels. Following surgery, parents and youth will complete electronic diaries regarding pain, emotions, medication use, and side-effects 3 times a day for 7 days and a 3-month follow-up survey assessing additional opioid use and opioid disposal. The central aims of this proposal are to understand parent and child factors that may predict opioid analgesic use following T&A procedures in order to inform predictive models for opioid-related outcomes following pediatric surgeries more broadly. A key innovation of this project is the examination of joint influences of parent and child factors on postoperative opioid use patterns. We hypothesize that parents and children who tend to catastrophize more about painful experiences, have positive beliefs regarding opioid efficacy, and less negative beliefs regarding opioid side effects will be more likely to use opioid medications and be less likely to dispose of opioid medications. Identifying these parent and child predictors could allow for the development of targeted screening and patient education in order to identify potentially modifiable targets for interventions to improve pediatric postsurgical pain management and reduce opioid-related risks. Further, this project provides critical career development support for a young investigator focused on developing expertise in pediatric acute pain, opi...