Smith, 74, has undergone intensive chemotherapy and radiotherapy regimes since being diagnosed with breast cancer last year.
"It leaves you so flattened. I'm not sure I could go back to theater work, although film work is more tiring. I'm frightened to work in theater now. I feel very uncertain. I haven't done it for a while," Smith told The Times of London. "It's one of those things you ought to keep on doing and I haven't for a bit. I would love to be able to because I do love it, but I feel a great lack of confidence."
The celebrated stage actress is also known for her work in the films, "The Prime of Miss Jean Brodie," "California Suite," "Hook," "Sister Act," "Washington Square," "Tea with Mussolini," "Gosford Park" and the "Harry Potter" blockbusters.
Smith told The Times she thinks the age at which she was diagnosed made it difficult for her to fight the disease.
"It knocks you sideways. It takes you longer to recover, you are not so resilient. I am fearful of the amount of energy one needs to be in a film or a play," she said. "The last couple of years have been a write-off, though I'm beginning to feel like a person now. My energy is coming back. (Expletive) happens. I ought to pull myself together a bit."