No, they can't find this authority, regardless of what the Drug Warriors say--because it doesn't exist! The Constitution is VERY specific when it comes to naming the crimes that the federal government has the authority to punish...& the 10th Amendment leaves all other crime-fighting responsibilities to the states.
The federal government is BLATANTLY ignoring & violating the 9th Amendment by its persecution of medical marijuana users (& their doctors as well) in the states that have enacted its use under state law.
And of course you thus have no problem with 98% of the drug war that is being fought by the states?