Demi Moore is opening up about the high “intensity” of filming The Substance, her new body horror film directed by Coralie Fargeat.

The Feud star, who plays Elisabeth in the upcoming movie, revealed in a recent interview with the L.A. Times that she was diagnosed with shingles while on a break from filming one week.

“To give you an idea of the intensity, my first week that I actually had off, where it was just Margaret working, I got shingles,” Moore shared. “And I then lost, like, 20 pounds.”

Shingles is a “painful rash illness,” which can be contracted when “the varicella-zoster virus (VZV), which causes chickenpox, reactivates in their bodies after they have already had chickenpox,” according to the Centers for Disease Control and Prevention.

Her co-star, Margaret Qualley, who plays Sue, also faced her own challenges throughout production given the film’s weight. “Oh, yeah, I had crazy acne for a full, long-ass time,” she added.

However, both stars knew Fargeat was going to push them to new levels with their roles in The Substance and they each fully embraced it.

“You have to walk away feeling that you put it all on the table,” Moore explained. “It called for it and it’s what you want to bring to it.”

The movie, a follow-up to the director’s 2017 debut feature Revenge, follows a fading celebrity who decides to use a black market drug, a cell-replicating substance that temporarily creates a younger, better version of herself.

“It’s really what she’s doing to herself that’s most violent,” Moore said of The Substance, which is set to hit theaters Sept. 20. “[The script] took something that is a very internalized violence against oneself and externalized it in this way that allows the audience to have a little objectivity and to then really see what we’re doing to ourselves through that harsh, constant criticism and comparison.”

Fargeat added, “I read a tagline in an article about the film recently that said, ‘Being a woman is body horror.’ The movie can be scary on many levels, but the first is about playing with the violence of what we do to our bodies.”