A patient with major depressive disorder is currently taking phenelzine. The psychiatrist is considering adding fluoxetine due to inadequate response. What is the most significant risk associated with combining these two medications, and what is the recommended management strategy?

Explanation

Combining MAOIs (like phenelzine) with serotonergic agents (like fluoxetine, an SSRI) carries a high risk of serotonin syndrome, a potentially life-threatening condition characterized by mental status changes, autonomic hyperactivity, and neuromuscular abnormalities. Due to the irreversible nature of phenelzine and the long half-life of fluoxetine (and its active metabolite norfluoxetine), a sufficient washout period is crucial. For switching from an MAOI to an SSRI, a washout period of at least 2 weeks is generally recommended for the MAOI to allow for regeneration of MAO enzymes. For switching from fluoxetine to an MAOI, a washout period of 5 weeks is needed due to fluoxetine's long half-life.