Fatal–nonfatal comparisons of the 2023–2024 US overdose decline depend on the emergency department case definition: a retrospective ecological analysis

US deaths involving synthetic opioids fell sharply in 2024. If deaths fell faster than nonfatal overdoses, it is tempting to conclude that each overdose became less lethal. Using CDC death and emergency department (ED) surveillance for fixed sets of jurisdictions, we find that in 2023–2024 deaths fell proportionally more than ED discharges under both an any-opioid and a fentanyl-coded definition, but whether 2023–2024 departs from earlier years depends on the definition. The fentanyl-coded share of opioid ED discharges rose from 11.5% to 26.0% after fentanyl-specific diagnosis codes were introduced in October 2020. Aggregate surveillance does not identify a change in per-event lethality. Retrospective ecological analysis; preprint, not peer reviewed. Analysis code is included.