Despite growing evidence to the contrary, researchers continue to posit causal links between cannabis, crime, psychosis, and violence. These spurious connections are rooted in history and fueled decades of structural limitations that shaped how researchers studied cannabis. Until recently, research in this area was explicitly funded to link cannabis use and harm and ignore any potential benefits. Post-prohibition cannabis research has failed to replicate the dire findings of the past. This article outlines how the history of controlling cannabis research has led to various harms, injustices, and ethical complications. We compare commonly cited research from both the prohibition and post-prohibition eras and argue that many popular claims about the dangers of cannabis are the result of ethical lapses by researchers, journals, and funders. We propose researchers in this area adopt a duty of care in cannabis research going forward. This would oblige individual researchers to establish robust research designs, employ careful analytic strategies, and acknowledge limitations in more detail. This duty involves the institutional recognition by funders, journals, and others that cannabis research has been deliberately misconstrued to criminalize, stigmatize, and pathologize.