Figure 2: Dextromethorphan biotransformation pathway. Dextromethorphan metabolism is largely dependent on the cytochromes P450 2D6 and 3A4 in children and adults. The cytochromes P450 2D6 is fully developed by one month of age. CYP2D6 indicates cytochrome P4502D6; CYP3A4, cytochrome P4503A4; UGT; glucuronosyltransferase.