Compounds and combinations thereof for treating neurological and psychiatric conditions

A method of treating agitation associated with Alzheimer's disease, comprising administering a dosage form once a day or twice a day, for at least 8 days, to a patient in need thereof, wherein the dosage form comprises a combination of dextromethorphan, or a pharmaceutically acceptable salt thereof, and an inhibitor of dextromethorphan metabolism in the human liver, wherein the dose of the inhibitor of dextromethorphan metabolism in the human liver is adjusted to achieve an AUC0-12 of dextromethorphan on day 8 that is about 300 ng*hr/mL to about 1,686 ng*hr/mL, and wherein the CMAI score of the patient is reduced at 5 weeks after starting administration of the dosage form.