Compounds and combinations thereof for treating neurological and psychiatric conditions

A method of treating agitation associated with Alzheimer's disease in a human patient using an N-methyl D-aspartate (NMDA) receptor antagonist, comprising administering a combination of the NMDA receptor antagonist and a second agent that increases the plasma levels of the NMDA receptor antagonist to the human patient, wherein the NMDA receptor antagonist is dextromethorphan in the free base form at a dose of about 33 mg twice daily, or a molar equivalent amount of a salt form of dextromethorphan, and the second agent is bupropion in the free base form at a dose of 91 mg twice daily, or a molar equivalent amount of a salt form of bupropion, wherein the human patient experiences a reduction in agitation symptoms after one week of treatment that is greater than what would be achieved with a placebo, wherein the human patient has a history of drug abuse, wherein the human patient does not experience depersonalization or derealization disorder, wherein the human patient is monitored for any indication of clinical worsening, and wherein the dextromethorphan and the bupropion are the only therapeutic agents in the combination.