One account of a morality of drugs would take drugs as chemicals that physically change us from one state to another. Our moral evaluations of them would thus turn on what state we started with, what state we ended with, and the reason for changing them.
This seems vacuous, but it might do real work. Consider a limited theological argument for why Christians should condemn the use of hallucinogens for religious or quasi-religious reasons (like the belief that knowledge of higher truth comes from taking them). For a Christian, the highest possible revelation is one given in the everyday world: it is a man walking around who is also God. Whatever mythical or theological elements might have worked into the story, they rest on a fundamentally and essentially historical basis. The hallucinogenic shift of consciousness is thus an implicit rejection that the highest, paradigm case of a revelation is given in everyday consciousness, and so a rejection of Christian revelation.
A non-christian analogue to the argument could be based on any claim that the space of revelation is the everyday world, and so, for example, should be based on the scientific or philosophical.