You make a good point, namely that it's not possible to trust an individual's testimony when under the influence of a substance. In this case, you or I may well be an outside observer, seeing that the person's ability to perceive the world is impaired. To the impaired person, however, their experienced reality must be true; without being told otherwise, they would have no reason to assume anything else.
I'm now thinking of a society where the majority of people consume a variety of substances which impair their perception of reality. In that scenario, the majority of people believe their interpretation of reality is true. The unimpaired knows those people are incorrect, but the impaired don't know better.
But this case creates a potentially uncomfortable situation for us. If some substances impair, what's not to say other substances enhance? If we know both performance-enhancing and performance-inhibiting substances exist, it stands to reason that both perception-enhancing and perception-inhibiting substances also exist. We know the latter exists, and our primary issue is that it's presently claimed the former does not. A performance-enhancing substance allows an athlete to perform beyond normal limits, and it stands to reason a perception-enhancing drug would do similarly.
However, what if a group of people are under the influence of an identical substance and, at the same time, observe real things which the "unimpaired" do not? The tricky word in the previous sentence is "real": what is real depends on your vantage point. The shadow of a cylinder may be either a rectangle or circle; to only debate the difference between which shadow is "real" obscures the true nature of the illuminated object.
So, a line of scientific inquiry may be the following: is it possible for a mind-altering substance modify an observer's vantage point of the universe? In a more illustrative framing, can a person in Plato's Cave eat something to see beyond their slice of reality? I don't know, but I believe the question is worth consideration.