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From SNPedia

Nuedexta is a trade name for an FDA-approved drug primarily used to treat Pseudobulbar Affect (PBA), a condition "characterized by involuntary, sudden, and frequent episodes of laughing and/or crying." Nuedexta contains a combination of dextromethorphan and quinidine.Nuedexta

The following excerpt is from the pharmacogenomics section of the EMA's label for this drug:

The quinidine component is intended to inhibit CYP2D6 so that higher exposure to dextromethorphan can be achieved compared to when dextromethorphan is given alone. Approximately 7-8% of individuals of Caucasian descent, 3-6% of Black African descent, 2-3% of Arab descent and 1-2% of Asian descent generally lack the capacity to metabolize CYP2D6 substrates and are classified as Poor Metabolizer (PMs). The quinidine component is not expected to contribute to the effectiveness of NUEDEXTA in PMs, but adverse reactions of the quinidine component are still possible.

Approximately 1-10% of individuals of Caucasian descent, 5-30% of Black African descent, 12-40% of Arab descent and 1% of Asian descent exhibit increased metabolic activity for CYP2D6 substrates and are classified as Ultra-rapid Metabolizer (UMs). In such UM patients, dextromethorphan is rapidly metabolised, leading to lower, potentially subtherapeutic concentrations.