The difficult-to-detect substance is so powerful that an amount equivalent to a few grains of salt can be deadly. It requires more aggressive treatment than a typical opiate overdose to reverse. First responders are getting burned out answering back-to-back overdose calls rising because of carfentanil and other synthetic opioids, and they worry about falling ill after exposure while answering calls. The drug is so new that some medical examiners don't have the tools to detect it in autopsies.