The change in national attitudes towards cannabis and in legal access to marijuana around the US over the past several years is staggering.

As of last fall, 57 percent of adults in the US said that marijuana should be legal, with only 37 percent taking the opposing view, which is essentially a reversal of the opinions held just a decade ago.

And after November's elections, a full 20 percent of the US population lives in a state that has voted to legalise recreational use - and far more live in states with some access to medical marijuana.

But this obscures a crucial fact. From a scientific perspective, there's still a ton we don't know about cannabis.