Every adult has an inalienable right to perform any act that doesn't violate anyone else's rights; doing drugs does not violate anyone else's rights (except in the case of parents who render themselves incapable of meeting their obligations to their children, but banning drugs for all adults is no better a solution to this problem than banning alcohol for all adults would be).
Inalienable rights are those God-given individual rights which cannot be taken away without due process in a court of law.
Smokin' dope ain't one of them.