David Nutt has a good piece: Here’s Why We Hear So Many False Claims About Cannabis
On the whole, my many years of research on substance use has taught me a major overarching lesson: we are much more likely to demonize drugs for their negative effects than consider their neutral or potentially positive impacts. Or – in scientific terms – there is a built-in bias in the scientific literature, textbooks, and the popular press towards highlighting the negative aspects of drug use. And more ink has been spilled about cannabis than any other drug, perhaps because it’s the most widely used illegal drug and the subject of intense debate concerning its regulation.
He goes on to talk about the ways in which funding and confirmation bias (although he doesn’t use that term specifically) affect the scientific results we hear or notice.