I'm not referring to drugs that harm the body. I am asking about someone who uses opioid pain killers to feel "high" and as a coping mechanism. The route of administration is not dangerous like needles or smoking.
In my understanding, Mutar and Asur are not dichotomous values of the Halakha, rather it is a long scale ranging from very bad explicit sins to highest levels of Kiddush Hashem.
Depending on person's spiritual level on this scale, all behaviors are valued as either Isur or Heter, relatively to his level. For example, using opioids for recreation for a Torah scholar would fall under Bitul Torah and be seriously Asur, but for one who use it to stay away from adultery or eases keeping Shabbos, that can turn into not only Mutar but almost a Mitzvah.
My point is, that as smoking (or other use) is not an explicit prohibition, it must be seen relatively to one's spiritual level.