Neurological medicines are extremely complex - while massively increased appetite isn't a usual side effect of meds like Methylphenidate it can cause eating disorders in the other direction due to an inability to perceive fullness and I have a close friend who suffered through this.

Having had other close experiences for the med balancing for someone with bpd and a complex host of other disorders - most of the times neurological med families are just trialed until one sticks and the specific efficacy and side effects of each can vary wildly from person to person. Brains are really complex and we don't really have an understanding of the method of action (specifically - why the med works - what it is changing in brain chemistry and why that has the impact we observe) of most neurological meds.

We even still struggle to comprehend specific biological markers linked to certain disorders - probably in part due to the fact that we're categorizing those disorders by wide swathes of symptoms.