The HBr is HydroBromide, which is the salt the DXM is bound to. It is in no way an active ingredient, but some salts do affect the absorption rate of medicines, but I think think this is the standard for DXM (not like it's extended release or anything). The USP just means that is a standardized dose - i.e. it's 15 mg of DXM, not 15 mg of DXM + HBr.



Or I could be completely wrong.