I don't know what you saw, but quinidine is NOT on any store shelf. It is a very strong heart drug, in fact not much used any more. It is in the Nuedexta because it impairs the metabolism of DM and keeps it in the blood stream a long time.
Nuedexta contains both DM and quinidine (which is supposed to be low dose).

Robert's experience is for PD. He thinks other ingredients like antihistamines combined with DM change its action.
I have found that DM helps my PN pain in my feet. It is not a huge effect but does help. I use a product to help clear my sinuses called Mucus relief DM... 1/2 tablet at bedtime.
I get it OTC at WalMart, at a reasonable price. It also has guaifenesin in it to help dissolve thick mucus.