The government is objecting to the use of controlled substances to assist suicide -- that doctors in the State of Oregon are using drugs for purposes not intended, nor approved, by the government.
And exactly how a drug gets used should be the decision of the federal government, not the state governments because...?
It seems to me that exactly how an individual and his doctor decide to practice medicine is more among the objects which, in the ordinary course of affairs, concern the lives, liberties, and properties of the people, and the internal order, improvement, and prosperity of the State, and thus a state government concern.
So Oregon can permit assisted suicide, and it can permit these drugs in other applications, but it can't permit these drugs to be used for assisted suicide.
But the feds aren't prohibiting assisted suicide, because you said they can't do that. They are merely prohibiting an unintended and unapproved use of a drug, and that use just happens to be assisted suicide, but it's not the assisted suicide that's being prohibited, it's exactly HOW they are assisting suicide that's being prohibited.
And if they don't assist suicide just right, interstate commerce will be impeded, so this is not just the business of the state of Oregon, but it's a federal matter.
Have I got it right?