I didn't like these two particular aspects...

1) I think they should have shared memories/experiences, something along those lines. If you're Demi Moore, why would you care so much if someone else was using a part of yourself to have fun? Demi didn't experience the things the other body was doing, didn't have any memories of it...why keep giving up parts of your body and your valuable time to essentially show a younger version of yourself a good time? This only makes sense if Demi gets something out of the deal. She still lost her job...she was miserable and eating all the time...she wasn't getting laid or out partying...she wasn't accumulating fun memories...I hardly see why she'd keep this up, especially after losing a finger.

2) Early in the movie, I thought for sure the hook was going to be that in order to continue living the fun/young lifestyle, the company producing The Substance would want more and more money. But they didn't seem to have any financial gain whatsoever out of the deal! Again, like Demi, I'm not sure what their motivation might have been. Why do they exist? They apparently pay for and maintain a staff, a 1-800 number, a storage unit, and a delivery system. I'm sure doctors/scientists to come up with the drug would have to get paid big dollars as well. And for what? Just to screw over a random person? Who's funding this and why?