Part of the reason for the surge in deaths is that many users do not know exactly what drugs they're buying and using, and in some cases they are inadvertently ingesting lethal amounts of synthetic opioids, which include fentanyl and carfentanil, an animal tranquilizer that has killed scores of people. The substances often are cut into heroin or pressed into counterfeit pills that users believe are painkillers or other drugs. The synthetic drugs also are proving to be resistant to overdose-reversing substances such as Narcan.