> Loperamide is not unique. I don't know why these researchers have singled it out.

Probably because it's what they were studying which led to the analysis? A drug doesn't have to be unique to be important in understanding side effects...or off-label use if that's how you want to think of it.

Getting funding to study all of them at once is more challenging, so it shouldn't be a surprise that it's constrained to one.