Demi Moore's character in The Substance takes the notion that ageing is worse than dying to a new - and horrifying - level

Would you take a miracle drug that guaranteed eternal youth? That granted poreless skin and supple limbs? That reversed all the violence of the world and restored the body to its smoothest, its tautest, its lithest?

What if it wasn't so eternal after all? What if it came with a raft of unknown - and grotesque - side effects? What if the flesh rotted and bruised, peeled off like mandarin skin? Would you keep taking it? Would you?

This is the Faustian bargain that powers The Substance - French director Coralie Fargeat's fantastically nasty, proudly gaudy piece of body horror that instantly cleaved audiences into loud, warring factions at its Cannes premiere earlier this year - and then won the festival's best screenplay award.

Demi Moore is the film's beating heart - and its sinew, bone and tendon - as Elisabeth Sparkle, a one-time Hollywood idol now suffering the consequences of an industry where ageing is worse than dying.

Fargeat's world is halfway between the 1980s excess of Wall Street and the neon dystopia of countless sci-fi ventures. It is a world where network television still reigns supreme and audiences still tune into a Jane-Fonda-lite aerobics show fronted by Sparkle in shimmer tights and a megawatt grin. Then, she is unceremoniously axed on her 50th birthday.

This is not a subtle film by any measure. A lecherous, chauvinistic studio exec called Harvey (Dennis Quaid) ushers Sparkle out of the building faster than you can say "Weinstein". Seconds later, Sparkle drives past her own obsolescence on her way home: a billboard of her beaming face being torn down.

The former leading lady gazes up at her past glory. Her face in the sky falls to the pavement in one long shred. Then the scene explodes: another car T-bones Sparkle's in a deranged chorus of ear-splitting screeches.

The ground seems to quake so violently you might wonder if you've accidentally entered a 4D session. It is a brutal, almost excruciatingly long sequence - though it's a stroll in the park compared to the rest of the film's hellish delights.

In the aftermath, a doctor slips Sparkle a USB containing an ad for The Substance: a mysterious panacea promising "a better version of yourself". Like any horror heroine, Sparkle immediately dispenses with any reasonable logic and succumbs to temptation.

Soon enough, she's staring at a highlighter-hued elixir and all manner of sharp objects: syringes, scissors and sewing needles. She "activates" by injecting The Substance and her spine ruptures to birth a glistening creature onto bathroom tiles. As the USB claims: it's Elisabeth, only "younger, more beautiful, more perfect" - and now played by a dead-eyed Margaret Qualley (Kinds of Kindness, Poor Things).

This new specimen calls herself Sue and quickly usurps her counterpart's life, replacing her television spot and swanning around her palatial apartment.

The rules of engagement are clear from the outset: Elisabeth and Sue are one brain split across two bodies. Only one can be conscious while the other lies dormant - and they must swap every seven days.

Rules, though, are designed to be flouted - particularly when you're Tinseltown's latest discovery and there's always another party, promotion and Vogue cover on the horizon. In a blink, Sue's seven days become eight, then a month, then three months. Elisabeth, meanwhile, rapidly decomposes. It's a feat of physical acting from Moore, covered in head-to-toe prosthetics and unleashing an animal ferocity after a decade of mostly thankless movie roles.

Fargeat's targets are many and varied. There is, of course, the meta-commentary on Hollywood's treatment of actresses who dare to - shock horror - age. There's the satire of our age-old pursuit of beauty at all costs - like Dorian Gray meets Botched. There's the skewering of Big Pharma and all its magic pills.

Trumping its polemics, however, is the film's commitment to spectacle. Each sound - an olive plunging into a martini, the crack of a soft drink can (or a bone) - is amplified to an arena-ready roar. The camera gawps at skin and saliva with frenzied glee; there's nothing more disgusting in The Substance than the prawn shells flung into a fish-eye lens from Harvey's squelching teeth.

It is pure pageantry. Go in with an empty stomach.

The Substance is now showing in cinemas.

- This story was first published by ABC News.