The link is to the full text of the article. The method (difference in differences) is well-known from the famous study on minimum wage laws (Card and Krueger,

The link is to the full text of the article. The method (difference in differences) is well-known from the famous study on minimum wage laws (Card and Krueger, 1994). > To examine the effect of cannabis access laws on opioid prescriptions, we estimate a series of difference-in-differences models, exploiting the staggered adoption of cannabis access laws over time ... Following Bradford et al. (2018); Buchmueller and Carey (2018) and Patrick et al. (2016), we include controls for [several] different legal changes that may impact opioid prescriptions in a series of robustness checks ... As described in detail in the appendix, we test our data for parallel trends between providers in states that adopted cannabis access laws and those in states that did not. We are unable to reject the null hypothesis of parallel trends, which supports the use of difference-in-differences models.