Unlike traditional horror films that rely on external threats, The Substance shifts the terror inward, making the protagonist both victim and villain. Horror has long been a vehicle for exploring human psychology—films like Black Swan and The Babadook use horror to externalize mental illness. The Substance follows in this tradition but takes it further by making self-image the very thing that haunts its protagonist. Beneath its themes of societal pressure, the film presents a compelling reflection of body dysmorphic disorder (BDD), a mental illness affecting about 3% of the population.