Wow.
"Intense" would be an understatement. HUGE understatement.
The film is quite incredible (like, "8.0" incredible), but less in a pleasant way and more in a stupefying one!
Ultimately, The Substance is a painful, brutal, and exceedingly graphic allegory about obsessing over youthful beauty at any cost.
If you are more than, say, 30% squeamish, you won't endure, so just take a mulligan on this one.
Our protagonist (an aging movie star named "Elisabeth," played rightly by one Demi Moore) goes to some extraordinary lengths to maintain her good looks, and I'd rather not say what exactly she loses in the process.
For whatever it's worth, I will say The Substance is definitely categorized accurately as "Horror" (or "Body-Horror," to be even more precise).
I could not help but think of the real-life Joseph Carey Merrick, characters Jack Griffin/Sebastian Caine, Henry Jekyll, Bruce Banner, Michael Morbius, Seth Brundle, and one infamously peculiar medical experiment that occurred at Ingolstädter Alte Anatomie in November 1792 or 1793, as The Substance's narrative arc flows through similar veins and its essence shares similar DNA.
