What is going on with Philips Zoom's Daywhite teeth-whitening formula? Excruciatingly painful; not the same as Discus Daywhite.

I've been happily using Discus Daywhite 9.5 teeth whitening gel for many years. It was mildly uncomfortable but not painful and I easily finished the regular recommended maintenance touch-ups on my own after first having it done professionally. The new product (Philips Zoom) is supposedly the same formula but it is excruciatingly painful. Cannot use it and won't because I know teeth-whitening doesn't have to be torture. What is going on??? I'm not the only person who's had this experience...