“Drug use” – leaving aside the therapeutic meaning and sticking with the illicit one – means at least four things:
1.) Use of hallucinogens, or drugs that are mind-altering at any dose, and not just with extreme dosages.
2.) Drugs that are not mind-altering at every dose. Alcohol is certainly included here, and some try to argue that cannabis is also.
3.) Performance enhancing drugs.
4.) Drugs that are addictive though only very loosely mood altering – caffeine and nicotine, etc.
Given the wild variety here, the only common criterion that people find to treat the moral question is the affects that drugs have on health. The criteria is very broad, but not very deep. It has the value of being a relatively clear (major health affects – e.g. overdoses and long term diseases are easy to see) but it suffers from at least three major limitations:
a.) It’s difficult if not impossible to get any significant health effects out of a single use of any drug, or even out of multiple uses over large enough stretches of time. Single uses are harmful only when we shift from talking about drug use to talking about poisons. But poisoning is not anything peculiar to drugs. One can be poisoned by anything ingestible.
b.) It leaves one with the impression that there is no moral significance to anything formally belonging to the drugs themselves. Is it wrong to get high? Always? Sometimes? Is cheating the only problem with steroids in sport?
c.) It sees the morally significant aspect of drugs as their relation to overdose or addiction. Heroin and alcohol (now seen as comparable, if not equivalent) are bad and pot is harmless. We spoke of overdose in (a), but addiction is much the same. We can become addicted to any concrete, repetitive activity that is contrary to reason.
To be honest, I’ve heard almost no moralities of drug use, even under a vague description. I’ve seen moralities of the ingestible and the repetitive applied to drugs, and I’ve heard people appeal to principles like “poisoning is bad”, but not much else. Even taking into account the trickiness of the virtue of temperance, one might expect more than this.