[SixSigma]

No. You will be charged with "possession of a controlled substance" in the UK even if the substance was not a controlled substance just so long as they can demonstrate you thought it was. E.g. the cops can set up a "drug deal" to sell you cocaine, sell you talcum powder and then arrest you for buying it, so long as they can demonstrate you think it is cocaine - such as saying "here's that cocaine you wanted".

[wil421]

Same is US, if you sell fake drugs you will be prosecuted as if you sold those drugs.

[mikeash]

This seems fair, especially if we want the opposite to be true: if you buy what you legitimately think is talcum powder, and it ends up actually being cocaine, that shouldn't be a crime!