I had to think of Requiem for a Dream and the story of the mother that wants to fit into her dress. She goes to the doctor, he gives her speed (I think?). She realizes that she is more efficient. Cleans the house quicker, does chores etc. and is even enjoying it. All the while she is losing weight! Perfect!
The day comes where she finally fits in her old dress... But she also notices she lost her mind.
My memory is hazy but did she get paranoid/psychotic at some point?