There is stalking, a character is sent messages from her own phone showing pictures of herself in her home, the stalker also shows up outside of her house multiple times throughout the film
Though the movie doesn't reveal this until the end of the scene, it only happens in the protagonist's imagination: she runs from the back door, he catches up, and he graphically, gorily smashes her head in. Her head is off camera during this, but gore flies into view. Her bloody head is seen after he stops.
Relatively late in the movie, the protagonist's right hand gets brutally smashed; it's never covered up, so from this point on, it's often on screen, her bloody fingers broken and bent at different angles. It's a disgusting (if not all too real-looking) prosthetic.
So "anxiety attacks" aren't a clinically recognised thing, but in the casual, natural language sense: There are *absolutely* anxiety attacks in this film. That's the whole point of this kind of suspense thriller.
the main character is deaf so her phone flashes when she gets a call and her fire alarm is a bright strobe light that goes on for a couple minutes two separate times in the movie.
the film itself is arguably ableist at points in regards to the main character being deaf, played by a hearing woman. some comments in the movie seem off color to me. this is a point of controversy so it's up to interpretation.
The protagonist is presented as the author of Midnight Mass(another story Flanagan adapted), which is a story heavily revolving around Catholicism, a cult, & philosophies/beliefs in life after death.
The book cover is shown on screen with a cross.
Early in the film, when she is working on her laptop, we are shown several of her ideas for endings to her next story, in which Jesus is mentioned.