A 70-year-old woman has a complex medical and a current drug regimen that includes calcium and vitamin d supplements for osteoporosis, metformin (glucophage) for type 2 diabetes, phenelzine (nardil) for depression, and metoprolol (lopressor) and furosemide (lasix) for hypertension. The woman is requesting dextromethorphan for the treatment of a recurrent cough. What component of her drug regimen contraindicates the use of dextromethorphan?