[andrewstuart]

My theory is that if you can't get a cheap, legal high then you are more susceptible to trying something else.

[eru]

Could you think of a way to test this?

(I imagine one can probably find lots of `natural experiments'. https://en.wikipedia.org/wiki/Natural_experiment)

[vidarh]

We test that every day: People get their oxycodone prescription etc. taken away.

Today this is one of the most substantial gateways to heroin use. A proportion of those who have their opoid prescriptions taken away turn to buying more on the black market. High black market prices for drugs like oxycodone lead people to try heroin.