The primary objective of the current study is to investigate the safety with special emphasis on ECG effects, and tolerability of dextromethorphan hydrobromide monohydrate (2mg/mL syrup) in healthy male and female subjects following oral administration of 30 mg q.i.d. and 90 mg q.i.d. for 2 days followed by a single morning dose (extensive metabolisers of CYP 2D6) and for 10 days followed by a single morning dose (poor metabolisers of CYP 2D6).