As Sue thrives, Elisabeth deteriorates which makes her more dependent on Sue and reluctant to quit The Substance.
The film also works as an allegory for addiction.
Elisabeths life is split into two parts: her real self and Sue.

Demi Moore as Elisabeth Sparkle in ‘The Substance’.
Sue is the person Elisabeth is while using.
As Sue thrives, Elisabeth deteriorates which makes her more dependent on Sue and reluctant to quit the substance.
The film ends with Sara institutionalized and undergoing electroshock therapy which renders her catatonic.

The Substanceends with both Elisabeth and Sue reaching rock bottom.
She shows up to her old life unrecognizable and reviled by those who see her.
She cannot return to her substance-free life.

Elisabeth Sparkle (Demi Moore) begins to realize what the substance has cost her.

Sara Goldfarb (Ellen Burstyn) didn’t respect the balance.