Serotonin syndrome is a potentially life-threatening condition caused by excessive central and peripheral stimulation of
serotonin brainstem receptors, usually triggered by inadvertent interactions between agents with serotonergic activity. Evidence supporting an association between nonserotonergic
opiates, such as
oxycodone or
morphine, and
serotonin syndrome is very limited and even contradictory. In this case report, we describe a patient who developed serotonergic-adverse effects likely precipitated by an interaction between
morphine and
phenelzine. A 57-year-old woman presented to the emergency department with complaints of increasing
visual hallucinations,
restlessness,
photophobia,
dizziness, neck stiffness, occipital
headache,
confusion, sweating,
tachycardia, and
nausea over the previous week. On admission, her blood pressure was 185/65 mm Hg, and clonus was noted in the lower extremities. The patient was hospitalized 10 days earlier for
cellulitis of the left breast secondary to a left
mastectomy 5 months earlier, and a short course of oral
morphine was prescribed for
pain control. Her routine medications consisted of
aspirin,
atorvastatin,
bisoprolol,
clopidogrel,
gabapentin,
omeprazole,
phenelzine, and
ramipril. Supportive measures were initiated on admission.
Phenelzine and
morphine were discontinued immediately, leading to a progressive resolution of symptoms over the next 48 hours.
Phenelzine was restarted on discharge without further complications. Use of the Drug Interaction Probability Scale indicated a probable relationship (score of 6) between the patient's development of
serotonin syndrome and the combination of
morphine and
phenelzine. The mechanism underlying this interaction, however, remains unclear and warrants further investigation. Clinicians should carefully weigh the risk and benefits of initiating
morphine in patients taking
monoamine oxidase inhibitors or any other
serotonin-enhancing drugs.