Sounds like a defense of principle to me. Anyhow, now that there are places like Colorado, longitudinal studies should be easy. And I’d bet the price of a doughnut factory that yes, they will show pot wreaking measurable harm if consumed in sufficient quantity. But that points up the difficulty of measuring something that is in the shadows. It is hard to do. Far easier to measure something that is in the sunlight.
Sometimes you can measure usage through proxies. Here is an example of what I mean.
If product is being shipped in, it is getting used. Increases in shipments directly correspond to increases in usage.
We will probably be able to ferret out usage by looking at increases in welfare applications from here on out.