How does quinidine affect dextromethorphan concentrations when taken with Benylin Dry Coughs Night Syrup?

Quinidine inhibits CYP2D6 and can increase plasma dextromethorphan concentrations up to 20-fold when taken with Benylin Dry Coughs Night Syrup, increasing central nervous system adverse effects; if combined use is necessary, monitoring and possible dextromethorphan dose reduction are required.