Researchers cannot yet say with confidence why deaths have gone down. Experts have offered multiple possible explanations: increased availability of the overdose-reversing drug naloxone, expanded addiction treatment, shifts in how people use drugs, and the growing impact of billions of dollars in opioid lawsuit settlement money.

Some also point to research that suggests the number of people likely to overdose has been shrinking, as fewer teens take up drugs and many illicit drug users have died.

Two other theories recently joined the list.

In a paper published last week in the journal Science, University of Maryland researchers point to the drug supply. They say regulatory changes in China a few years ago appear to have diminished the availability of precursor chemicals used to make fentanyl.

Their argument is based partly on information from the U.S. Drug Enforcement Administration, which last year reported that the purity — and dangerous potency — of fentanyl rose early in the COVID-19 pandemic but fell after 2022. It suggests it became harder to make fentanyl and its potency was diluted.