BackgroundOpioid overdose continues to be a major cause of death in the United States. One effort to control opioid use has been to implement policies that enhance criminalization of opioid possession. Laws to further criminalize possession of fentanyl have been enacted or are under consideration across the country, including at the national level. ObjectiveEstimate the long-term effects on opioid death and incarceration resulting from increasingly strict fentanyl possession laws . DesignWe built a Markov simulation model to explore the potential outcomes of a 2022 Colorado law which made possession of >1 g of drug with any amount of fentanyl a Level 4 drug felony (and escalation of the previous law, where >4 g of any drug with any amount of fentanyl in possession was considered a felony). The model simulates a cohort of people with fentanyl possession moving through the criminal justice system, exploring the probability of overdose and incarceration under different scenarios, including various fentanyl possession policies and potential interventions. SettingColorado ParticipantsA simulated cohort of people in possession of fentanyl. MeasurementsNumber of opioid overdose deaths, people incarcerated, and associated costs over 5 years. ResultsWhen >4 g of a drug containing any amount of fentanyl is considered a felony in Colorado, the model predicts 5460 overdose deaths (95% CrI 410–9260) and 2,740 incarcerations for fentanyl possession (95% CrI: 230–10,500) over 5 years. When the policy changes so that >1 g possession of drug with fentanyl is considered a felony, opioid overdose deaths increase by 19% (95% CRI: 16–38%) and incarcerations for possession increase by 98% (CrI: 85–98%). Diversion programs and MOUD in prison help alleviate some of the increases in death and incarceration, but do not completely offset them. LimitationsThe mathematical model is meant to offer broad assessment of the impact of these policies, not forecast specific and exact numerical outcomes. ConclusionsOur model shows that lowering thresholds for felony possession of fentanyl containing drugs can lead to more opioid overdose deaths and incarceration.