> I would generally trust it if it persists after the actual pill wears off.
The problem I see with that is that the thing that you are using to "trust it" may have been altered by the pill itself, to make you "trust" whatever changes it made. It could be like Ken Thompson's Hack[1], but for brains.
On the other hand, that's how our brains work, regularly. We experience reality and our brains' chemistry changes.
A similar situation I can think of is: I have an accident and and I lose part of my brain mass as a result. The present me would definitively not "trust" whatever person results from that. But "me without those brain parts" might, or even like it. It has been observed that brain injuries can produce severe personality changes[2]
I must point out that a brain injury might feel more "drastic"; more similar to a "hardware change" in a computer than the pill one, which could feel like a "software update", of sorts. But a pill could very well kill a huge part of a brain (consider that cyanide could kill 100% of it).
[1] http://wiki.c2.com/?TheKenThompsonHack
[2] https://en.wikipedia.org/wiki/Phineas_Gage