I've seen this film twice. I'm going to try to not drop too many spoilers.
For me, this film is an allegory that dives into the dark side of ego, vanity, addiction, self destruction, and the internal power struggle and desperation when faced with aging and our own morality. It was more tragedy than horror for me. (The gore is the only element of horror.)
This film moved me to tears where it made other people in the theater laugh out loud. The main reasoning being that I have experienced the destruction of addiction and the parallels in my own life were jarring.
Elisabeth is me. Sue is me. When you are in the grips of addiction, you find an escape from your pain and over and over again, you reach for that relief until it consumes you. Against your better judgment, you continue to self sabotage to your mental and physical detriment. It was heartbreaking to witness the deterioration of the characters in the pursuit of "happiness" by way of abusing The Substance.
I haven't read every review but many of them seem to rely on the physical aspects of the film rather than the deeper messages. Everyone regurgitates the beauty standards theme but for me it was so much more than just skin deep.
It was a journey of self discovery through the eyes of the duality of one person. The vascilation between right and wrong. The internal battle to make the right choice when you cannot muster the willpower to make it. It also shows how quickly addiction can take hold of a person. It look about 4 months for everything to devolve into what it did.
Now let's address the gore. Warning: there is an IMMENSE amount of blood and some of the scenes toward the ending may be difficult to watch if you are squeamish. The short-lived rush of external validation created a monster and the blood was a purging of everyone's expectations of Elisasue. The blood was more alarming than frightening for me because of its sheer quantity.
I really enjoyed the film and I'm grateful for its existence. I hope that when people see it, they look past their own hang-ups and preconceived notions to find something meaningful. This film is polarizing for sure, but you should allow it to speak to your inner self. If you hate it, ask yourself why. Not to be patronizing, but the people who focus on the physical nature of the film lack emotional depth, or perhaps the ability to critique one's own ego. Have empathy for yourself and have empathy for others.
And remember, take care of yourself.