Opioid overdose deaths have increased more than fivefold from 1999 to 2016, accounting for 42,249 deaths in 2016.1 One particularly challenging aspect of the opioid epidemic is that it has been marked by a rapid transition from prescription opioids to heroin to synthetic opioids. This third wave involving synthetic opioids has largely been driven by illicitly manufactured fentanyl and its analogs.2

To address challenges of quickly identifying newly emerging synthetic opioids, novel data sources such as web or social media data may serve as potential early warning systems. Prior work has mainly focused on automated identification of messages indicating misuse,3 detection of online illicit pharmacies, evaluating opinions around certain compounds,4 understanding spread of norms, and comparing online findings to survey data.5 However, there is particularly limited work examining fentanyl and fentanyl analogs (now the leading cause of overdose deaths) and limited work that directly compares findings from these novel approaches to death data to describe how much lead time an early warning system based on online data could potentially provide. Thus, this retrospective analysis sought to assess the degree to which such an early warning system could have provided early insights about the rise in synthetic opioids deaths.