Please explain where the Constitution says that a state government cannot regulate narcotics.
The state governments can... but this is the result of the federal WoD, not some wild coincidence. Banning alcohol led to moonshine, banning cocaine led to crack, banning speed led to meth. Now they are going to ban Sudafed? Like someone isn't going to just mail-order a crate of the stuff anyway? Then what... search the mail to make sure none gets through?
"Please explain where the Constitution says that a state government cannot regulate narcotics."
That's an easy one. The Tenth Amendment.
"The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people."
The Constitution made provisions for just this type of action by the States -- not by the Federal Gov't per se, but definitely for the states.