Drugs are also not knowingly self-administered. No sane person, knowing they will be consigning themselves to a life of horror and misery would willingly do such a thing.
That is why they are so deadly. They are harmless seeming time bombs that end up maiming and killing people.
No, the Defense clause is quite appropriate.
Untrue for marijuana - according to figures cited by the federal government's Institute of Medicine, of all those who use it 91% never become dependent.