The Substance

On her 50th birthday, now-faded Hollywood film star Elisabeth Sparkle is dismissed from her long-running aerobics TV show by its producer, Harvey, due to her age. Elisabeth crashes her car while distracted by a billboard of herself being taken down. At the hospital, a young nurse slips her a flash drive advertising "The Substance", a black market drug that promises a "younger, more beautiful, more perfect" version of oneself.
Elisabeth orders The Substance and injects the single-use serum. She convulses as a younger woman, Sue, emerges from a slit in her back. The two bodies must switch consciousness every seven days without exception, with the inactive body remaining unconscious and fed intravenously. Daily injections of stabilizer fluid, extracted from the original body, are required to prevent Sue from deteriorating.
Sue becomes an overnight sensation after auditioning as Elisabeth's replacement, and is offered by Harvey to host the New Year's Eve show. While Sue lives a confident and hedonistic life, Elisabeth becomes a self-hating recluse. Near the end of one weekly cycle, Sue brings a man home for casual sex; she delays the switch by extracting additional stabilizer fluid, causing Elisabeth's right index finger to rapidly age. Elisabeth contacts the supplier, who warns that delaying the switching schedule will lead to irreversible, rapid aging of the original body.
Elisabeth and Sue gradually become separate personalities and begin to hate each other; Elisabeth resents Sue's frequent disregard of the switching schedule, causing further aging, while Sue is appalled by Elisabeth's descent into unhealthy binge-eating. Following a particularly destructive episode as Elisabeth, Sue stockpiles stabilizer fluid and refuses to switch back.
Three months later, on the day before the New Year's Eve show, Sue runs out of stabilizer fluid and contacts the supplier, who informs her she must switch back to replenish the fluid. When they switch, Elisabeth finds herself transformed into a deformed, hag-like hunchback. To stop Sue aging her further, Elisabeth orders a serum designed to terminate her. However, Elisabeth, still craving Sue's celebrity status, stops before fully injecting the serum and resuscitates Sue, leaving both of them conscious. Realizing Elisabeth's initial intent, Sue beats Elisabeth to death before leaving to host the New Year's show.
Without Elisabeth, Sue's body rapidly deteriorates. She attempts to create a new version of herself using the leftover serum, despite the single-use warning. This results in the creation of a grotesque mutated body, "Monstro Elisasue", with both Elisabeth and Sue's faces. Wearing a mask cut from a poster of Elisabeth, Monstro Elisasue attempts to host the show, but the audience erupts into chaos when her disguise falls off. An audience member decapitates her, only for an even more mutated head to grow back, and one of her arms breaks and drenches the audience and studio in blood.
Fleeing the studio, Monstro Elisasue collapses and explodes into viscera. Elisabeth's original face detaches from the gore, crawling onto her neglected star on the Hollywood Walk of Fame. She smiles as she hallucinates being admired before melting into a puddle of blood, which is cleaned up by a floor scrubber the next day.