First of all, an important detail: Chemistry.SE has a tag for "drugs", and Biology.SE doesn't (it has one for "pharmacology"). Yet, I am still a little uncertain, and considering this question could potentially be my only contribution to either of these sites, I'd like it to be well-fitting. My uncertainty arises from the fact that the question isn't so much about the chemistry of drugs, but rather the physiological effects it has on humans. Adding to this, many drugs are natural and not chemically produced. Being unfamiliar with both sites, I am not sure how set facts places it in terms of suitability.
Here's the question in case you need more info:
This site claims there to be seven different categories of drugs, two of whom include "cannabis" and "inhalants". I find this hard to believe, as I don't think cannabis constitutes as a fundamental drug category. My assumption was that it fell under one or more of the other categories, either stimulants or depressants, or both (if that's possible).
As for inhalants, its name seems to denote the method of consumption, and not the primary physiological effect it imposes (stimulants, depressants, etc.), nor the primary ingredient of which it is made of (opioids).
Just so we're on the same page, this is the list of the fundamental drug categories I believe there to be:
Dissociatives (Not sure about this one, wondering if it's just a sub-category of "depressants")
Opioids (Not entirely sure about this one either, as I think it might be a sub-category of "stimulants")
I understand the site I linked to might not be reputable enough to be even worth discussing over, but thing is, as I searched around I found similar trends, though perhaps differing specifics. Lists compiling sub-categories of drugs next to fundamental divisions of physiological affectation. So, I found neither a consensus, nor a list with consistent categorization of drugs. My belief is that many of these sites threw in some of the fundamental types and then added a few of the most common ones, like cannabis.
EDIT: I do realize that this question centers around the categorization of chemical substances, which probably seems to make it a solely chemistry question, but that categorization is (in my understanding) determined by its physiological effects, meaning it is really a question about the human body, or am I wrong?
EDIT 2: User Rob notified me of a site, Medical Sciences.SE, which also seems like a good candidate, especially from looking at its tags.