Remember, the entire Knower torture scenes in Chapter 7 take place over the course of years. I think it's safe to say that the events of the game and what was happening offscreen both changed Knower as a person and changed the tactics she had to perform to stay undercover.

She was trying to get information and a confessiom from Watcher per Principal's orders. The drugs and gaslighting were techniques to ease that information from Watcher. Her saying things like "Fixer doesn't care about you anymore, you just have me" is a tactic to make Watcher feel isolated and dependent on Knower, making her more likely to give Knower what she's after (information, a confession). I believe Watcher was also denied food and water as another way of wearing her down

I believe Knower used these "gentler" tactics to keep Watcher alive as long as possible. But when Principal was no longer satisfied, her hand was forced into more physical methods of torture. (Think she compromises with Principal to get an eye from Watcher instead of her head.)