-- "Where in the Constitution does it give the Feds authority to outlaw drugs?"
You must have missed the part about repelling enemies, setting standards, regulating interstate commerce, and collecting taxes.
None of those enumerated powers give governments the ability to ignore the Constitution in order to prohibit/'outlaw' property like guns & drugs. -- Did you 'miss' that?
Plus, and this is very important and gets to the heart of the matter, the drug legalization [anti-prohibition] people don't really want the government to remove the heavy hand of coercion from their necks ~ nosirreebob ~ they want it lifted up and placed on the rest of us to keep us from protecting ourselves against them.
Not true in that you can protect yourself from 'addicts' of every stripe by allowing them to over-use their easily available poison. Alcoholics die young when left alone.
That is written in the Second Amendment ~ the part where it says "it's OK to "bear arms" against any and all intruders, particularly dopers.