A 33-year-old woman comes to the physician because of a 3-day history of dry cough and low-grade fever. Four months ago, she was diagnosed with major depressive disorder and staed treatment with fluoxetine. Physical examination shows no abnormalities. A diagnosis of upper respiratory infection is made and medication is prescribed to relieve her symptoms. A drug with which of the following mechanisms of action should be avoided in this patient?

Dextromethorphan, an antitussive with serotonergic effects, increases the risk of life-threatening serotonin syndrome in patients taking fluoxetine.Dextromethorphan is an NMDA glutamate receptor antagonist that desensitizes the medullary cough center to signaling from cough receptors in the respiratory tract. It is also a serotonin receptor agonist and has nonspecific serotonin-norepinephrine reuptake inhibitor effects. For this reason, it increases the risk of serotonin syndrome when combined with other serotonergic drugs (e.g., fluoxetine) and should be avoided in this patient.