A good few years ago I dreamed I had a sister, and that she had been taken away from me in childhood. The dream was extremely vivid and potent, to the extent that when I woke up I was so thoroughly confused and distraught I almost called my mother to ask her what they did to her. Within that same confusion I debated with myself if I should rather call my grandfather first, since I trusted him. He died in 2005.
The mental and emotional anguish of losing my sister, the blurring of reality and dream, and the gradual realization of reality across some twenty minutes was an incredible and brutal gutpunch. I never had a sister.
Unlike you Americans I never took any sort of medication or other such nonsense. This happened when I was about 25 - 28, so a bit more than a decade ago. It remains my most significant dream.
The girl who was but is not, or the dead girl, or the girl who was but was taken away is a common shared experience among a... certain subsection of people. We discussed this shared experience extensively back before the Internet became what it is now. To be precise, the subsection of people that frequented old /pol/ before the orange turd. The occultist lot. The less I say the better.