It's a Headache from Hell.

Brian comes under the addictive spell of a parasite with the ability to induce euphoric hallucinations in its hosts.
In Brain Damage, consciousness is carried by monsters and creatures — a plane of “parasitic addiction”. The story fights on E8 of the E0–E14 reality ladder, compressing addiction, control, horror.
Films fighting at the same level of reality, about the same things.
Other films that move a human interior onto the same kind of thing.
Hellraiser: Hellworld (2005)consciousness in monsters · hellworld ritual
Nanny McPhee (2005)consciousness in monsters · magical Victorian household
Morbius (2022)consciousness in monsters · vampire noir
Truth or Dare (2018)consciousness in monsters · cursed truth‑or‑dare game
The Boogeyman (2023)consciousness in monsters · fear‑haunted house
Bedazzled (2000)consciousness in monsters · deal-with-the-devilThe same preoccupations carried by a different kind of thing — grief told through the dead, an animal, a machine.
Jexi (2019)consciousness in machines · addiction, control
The Matrix (1999)consciousness in machines · control
The Lego Movie (2014)consciousness in objects · control
A Nightmare on Elm Street (1984)consciousness in the dead · horror
The Host (2013)consciousness in aliens · control
Dark City (1998)consciousness in aliens · controlShared director or cast.