25/F and 12 months ago, I had Botox in my masseter muscles, now slight sagging skin and 'hamster' cheeks. Will this go away?

First, it is important to emphasize that Botox works on muscles and is not for shrinking fat or skin reshaping or recontouring. When used for masseter (chewing muscle) hypertrophy, which may be genetic (as in Asian women) or from teeth grinding (bruxism), Botox can help to diminish the hypertrophy of the masseter muscles that can make for a wider, bottom-heavy face. It will NOT work, nor is appropriate for treating skin laxity-related jowls or excess fat in the lower face. From what you have written, I would guess that masseter hypertrophy may not have been the primary issue. I would suggest that you consult with a board certified aesthetic physician to discuss possible treatment options such as the use of injectable volumizing and lifting fillers to whatever regions may need it--such as the cheeks, jawline and chin region--in order to create or reestablish the more ideal, heart-shaped face you desire. Best of luck.

its very likely that the skin laxity was present even before you got the Botox in the cheek muscles. and that it became rather pronounced after the injections. Im not sure if the laxity will improve after the Botox has worn off. you need to be evaluated in person. and it would be better to take a look at your photos before you had the Botox injections. 

12 months after your last Botox treatment, it is unlikely that the Botox you had injected into the masseters is responsible for the changes you see in your face. I recommend an in person consultation with an experienced aesthetic physician to discuss your cosmetic concerns and to formulate an appropriate treatment plan. Good luck.