The US has signed up to a treaty that requires us to treat marijuana as an illegal controlled substance. That requires an explicit act of Congress to abrogate, and no court is going to find otherwise.
It probably should, but you may be wrong about the weight of legal opinion: https://www.nbcnews.com/think/opinion/donald-trump-can-unila...
In any case, the relevant treaty is not self-executing, and there is no mechanism to force Congress to criminalize drugs if it chooses not to even without explicitly abrogating (or even the President unilaterally disavowing) the treaty. Foreign countries could try to bring a case against the US before the ICJ, but even if successful, that wouldn't actually force changes to domestic law.
This was the first result for both “drugs treaty” and “Narcotics treaty”.
Problem is, ever since Wickard v. Filburn decision, all that the federal government needs to do is to add "... in interstate commerce" in the text of the law, and it is magically constitutional. The words themselves have practically no meaning - in Wickard, the Supreme Court ruled that a person growing food for personal consumption is still "participating in interstate commerce", on the basis that they'd have to buy that food otherwise, and therefore affect market prices on it.
I always wondered about that. Has there been any chatter about someone trying to get a federal court to nullify the law? I have no idea how that can work.