Dextromethorphan/Quinidine Sulfate is part of the NMDA Receptor Antagonist / Antiarrhythmic Combinations class and treats Pseudobulbar Affect. NMDA receptor antagonist / antiarrhythmic combinations are used to treat multiple sclerosis and ALS. They work by controlling involuntary laughing or crying which is known as pseudobulbar affect (PBA). Dextromethorphan/Quinidine Sulfate is only available as a generic drug.