The U.S. opioid epidemic is being driven by an unprecedented surge in deaths from fentanyl and other synthetic opiates. Fentanyl’s powerful effects are long-lasting, and even tiny amounts of the drug can lead to an overdose. Antidotes, such as naloxone, do not last long enough in the body to fully counter the drug, requiring repeated injections. Now, scientists report that they are developing single-dose, longer-lasting opioid antidotes using polymer nanoparticles.