The MPAA rating of R stands for "restricted," and that seems on-point for a film like 12 Monkeys. Why the hard R? This film hits the trinity of parental warnings: swearing, violence, and nudity.
A small sampling of the film's offerings will illustrate our point:
All of the English language's curse words are on display.
Extinction, bum fights, and hotel-bathroom dentistry. Need we say more?
The film features pimps, hoes, and ladies mistaken as hoes.
We get to enjoy some Bruce Willis butt. It's not sexualized, as he's being deloused, but heads up all the same.
Oh, forced delousing. Ew.
Viewers also witness intense scenes throughout. For example, Brad Pitt's performance as Jeffery Goines is unhinged—in the best kind of way, mind you—and can really get under your skin. There are also several scenes detailing the suffering the homeless and mentally ill endure at the hands of their peers and aloof authority figures.
Given this, 12 Monkeys should probably be kept away from young'uns, those with keen sensitivities, and those who wish to maintain their grip on reality. For everyone else, grab your barrel organ and welcome to the madhouse.