For the most part the areas the federal government can regulate are limited by Article 1, Section 8. I don't see anything there about protecting people from their own idiocy.
Except one of the rights/responsibilities of the Federal government is protecting from outside invasion. Heroin is being used just like a chemical weapon against this country. These drugs are mostly produced by third world countries that our sworn enemies of us, illegally smuggled into this country, and our currency used to buy these drugs are sent back to these enemies. There is very little between this and dropping bombs on us.