The types of opioids implicated in drug deaths have changed over time. Scholars have described the pattern using the metaphor of ‘waves’. The first wave of overdose deaths was in the late 1990s or early 2000s, when there was a rise in deaths involving prescription opioids. The second wave began in 2010 and was linked to an increase in heroin-related deaths. The third wave began in 2013, with a shift to deaths involving fentanyl. And then, the fourth wave began in 2015 with a rise in deaths involving fentanyl combined with stimulants.