Experiences - What is this?!

[Abuse]

swim first tried diphenhydramine three days ago (700mg) and he has no idea at all what happened to him. swim sat in his room and talked with friends for a few hours and heard his mom come to his bedroom and knock, saying "are you okay?" over and over and over again. swim got up to answer the door and suddenly realized he was home alone and that nobody was ever there. he has what seems like a weeks worth of memories with his afformentioned friends ,and when swim brought one of them up yesterday they were like "dude, what the fuck are you talking about??" i've never been one for religion but when swim looked in his bathroom mirror and saw his darker side smirking at him and staring with blood-shot eyes he was never the same again. it's like it changed my views on reality and imagination and swim came to see that they're so close it scares swim

i'm not gonna go into that much detail about this experience but swim would like to know why this happens. how can a drug create hallucinations so strong that they are indistinguishable from the real thing? how can it creat false memories? and why did swim run from one side of his house to another trying to escape a shadow figure that was chasing him, all the while screaming at the top of his lungs and jumping over couches. it's like this drug possed swim. how can anyone explain this?