Do you know some anime where one of the characters experiences some form of hallucination?

All the time, or just once/once in a while?

Like, most of the time.

Or, in alternative, lives in some form of constructed paradise.

Do “trapped in an MMO” anime count?

I don’t like anime related to the internet.

Does being trapped in the afterlife / limbo count?

Yes.

Well, then how about “Angel Beats”, “Haibane Renmei”, and “Konohana Kitan”?

What about “being transported to an alternate universe”?

If this alternate universe is confusing, distorted, not quite “material”, as I would say, then yes.

Well, then you are on your own.

Beavis when he eats sugar. He goes psychotic.

If you count Disney cartoons as anime, my favorite would be the near acid experience of Dumbo.

How dare you.

Perfect Blue. The girl hallucinates so badly it’s hard to tell what is real.

“Paprika.”

I just found two: “EF – A Tale of Memories/Melodies”. Do you know others similar to that one?