Nah, obesity reduction is itself a downstream effect of messing with neurotransmitters. There have to be other consequences of that - both good and bad.
There don't have to be any other consequences, certainly not both good and bad ones. Biology doesn't actually have some scale of justice that means good things must be offset by bad things.
But yes, it's very probable (in fact we already know) the drug is doing several things in the body.
I was more thinking that evolved systems rarely have convenient switches you can flip and just get a single outcome. You perturb the system, and you get a cascade of changes. It's not like engineering.
Nothing about justice was implied, so lets say desirable and undesirable instead of good and bad.
