Now, it's being treated like an opioid (which it isn't).
No, it's not being treated like an opioid. It's being treated like the controlled substance it has always been. It is considerably less-regulated than opioids.
It's definitely not illegal to bring prescribed meds into Mexico
It depends on the medication. It is most definitely illegal to bring some prescriptions, and even some nonprescription drugs, into Mexico.
the one thing that can get people into trouble is OTC Benadryl (from what I have read)
No, diphenhydramine is readily available all over Mexico.
Pseudoephedrine, even in combination products and even with a prescription, is illegal to bring into Mexico or to have in your possession there.
If I bring anything like that to CZM, it's only one or two pills.
Again, diphenhydramine is fine. Smuggling in an illegal drug, even as "only one or two pills", can still get you into serious trouble.
I might get into a little trouble if I brought lorazepam back to the USA, but I doubt it if I have a prescription - even from a MX doc.
There's no "a little trouble" bringing illegal controlled substances into the US. However, it's perfectly legal to bring medication for which you have a prescription as long as the drug isn't banned in the US.