Please don't list this on a work's page as a trope. Examples can go on the work's YMMV tab.
Nightmare Fuel: Fear and Loathing in Las Vegas
The adrenochrome sequence is just... ugh.
There's also Ralph Steadman's illustrations in the book.
Dr. Gonzo with the knife on a bad acid trip.
Duke: One of the things you learn from years of dealing with drug people is that you can turn your back on a person, but never turn your back on a drug—especially when it's waving a razor-sharp hunting knife in your eye.
Dr. Gonzo in general.
The diner scene.
It's never disclosed what happened to the waitress to cause such a horrific flashback, but judging by Ellen Barkin's reaction in the film it was too gruesome even in an a film replete with drug experiences. It's even more horrible than the bathtub scene as Gonzo is completely lucid and she can't tell if he will murder her for not letting him buy the pie.
Duke is not given the chance to apologise for this, nor does he seem inclined to.