INTRODUCTION: Law enforcement seizure data can indicate potential shifts in drug availability. We examined trends in the size of illicit fentanyl seizures in the United States from 2018 through 2023. METHODS: Using national High Intensity Drug Trafficking Areas data, we examined trends in the weight of fentanyl powder and number of fentanyl-containing pills per seizure with a focus on federal and other common state-level weight cutoffs that indicate various levels of trafficking (e.g., 1-4 g, 40-399 g, ≥400 g). RESULTS: Between 2018 and 2023, the plurality of fentanyl powder seizures weighed 40-399 g (27.7%), followed by seizures weighing 4-39 g (25.8%), 1-4 g (18.6%), ≥400 g (17.4%), and <
1 g (10.4%). During this period, there was a decrease in the percentage weighing ≥400 g (annual percentage change [APC] = -5.3, 95% confidence interval [CI] -9.4, -1.0) and an increase in the percentage weighing 1-4 g (APC = 5.6, 95% CI 3.2, 8.1). With respect to fentanyl pill seizures, the plurality contained 40-399 pills (10 = 1 g
25.4%), followed by 400-3999 (21.8%), <
10 pills (20.4%), 10-39 pills (18.6%) and ≥4000 pills (13.8%). During this period, there was a decrease in seizures of <
10 pills (APC = -12.6, 95% CI -23.2, -0.7) and increases in seizures containing 400-3999 (APC = 14.0, 95% CI 5.3, 23.1) and ≥4000 pills (APC = 12.6, 95% CI 7.4, 17.8). DISCUSSION AND CONCLUSIONS: The size of fentanyl seizures varies greatly, with a sizeable portion legally classified as drug trafficking. While the weight of fentanyl powder seizures is decreasing, the size of pill seizures is increasing. The increasing availability of fentanyl pills in particular needs to be monitored to inform prevention and harm reduction efforts.