[gdani]

Drug Usage in the US is about 2% currently.

First, there are no reliable statistics regarding drug usage in the U.S. because the majority of illegal drug usage is underground.

Second, I don't suppose that figure of 2% deliberately excludes alcohol and prescription drugs? If so, why?

[DiogenesLamp]

First, there are no reliable statistics regarding drug usage in the U.S. because the majority of illegal drug usage is underground.

Reliable or not, those are the figures being put out by the people who make it their job to come up with such figures. No doubt they are a statistical extrapolation. Till someone presents an argument that there are better figures, I will have no choice but to use what is available.

Second, I don't suppose that figure of 2% deliberately excludes alcohol and prescription drugs? If so, why?

I would assume it does, and because those are legal.