Effects of Retinoids on CYP2D6 Activity During Pregnancy

Inclusion Criteria: * Singleton pregnancies * Extensive metabolizers Exclusion Criteria: * Chronic or persistent cough accompanying asthma, smoking or chronic obstructive pulmonary disease * Productive cough * Fever * Known kidney disease * Known liver disease * Diabetes * Obesity * Bipolar disease * Attention deficit disorder * Social phobia * Allergy or history of adverse event with dextromethorphan * Drugs, foods, beverages or dietary supplements that interact with CYP2D6 or dextromethorphan * dextromethorphan * Unable to give written informed consent * Inability to fast * Concurrent use of vitamin A supplementation