Just because ADHD causes other problems doesn't mean that ADHD itself comes out of nowhere. The previous post has a valid point that ADHD itself is a symptom of something else and that drugs aren't necessarily the best way to reduce/eliminate ADHD at the source.
Is that actually understood, though? We had the heavily promoted social consensus for a long while that depression is caused by a chemical imbalance, which is now seen as a somewhat disproven theory that never had much evidence.
From a video I linked elsewhere in this discussion:
Gallo, E. F., & Posner, J. (2016). Moving towards causality in attention-deficit hyperactivity disorder: overview of neural and genetic mechanisms. Lancet Psychiatry, 3(1), 555-567. Konrad, K., & Eickhoff, S. B. (2010).
Is the ADHD brain wired differenctly? A review on structural and functional connectivity in attention deficit hyperactivity disorder. Human Brain Mapping, 31(1), 904-916.