If we can see the value in chastity, we can see the evil of drug use: just as artificially altering impeding the function of one’s reproductive organs is a great evil, so a fortiori is artificially altering and impeding ones organs of thought and reasoning.
I can alter the natural disposition of my consciousness; I can alter the natural object of my consciousness as present in mind. But the natural object of my consciousness is truth. Therefore if drug use is good, error and falsehood are as well.
Truth consists properly in conformity to mind. The intentional and deliberate desire to alter consciousness is therefore a desire to alter truth- it is in a very real sense a rejection of all truth as such.
To seeks an artificially altered consciousness requires that we find or natural consciousness lacking. But our natural consciousness is has as its first connatural object being. Drug use, then, involves some rejection of existence, and a judgment calling it insufficient.
The use of psychiatric drugs also proves the same point: the purpose of drugs acting on the mind is to return the mind to its natural state. Again, the very moral measure of the drug is how well it can move a damaged consciousness to its natural state.