A 37-year-old woman is brought to the emergency department 30 minutes after the onset of a severe occipital headache, palpitations, and tremulousness. Shortly before this episode, she ate several slices of pepperoni pizza. She has a 2-month history of atypical major depressive disorder treated with phenelzine. She is otherwise healthy. She appears anxious and tremulous. Her temperature is 37.8°C (100°F), pulse is 94/min, respirations are 20/min, and blood pressure is 210/110 mm Hg. Physical examination shows no other abnormalities. The most appropriate next step is administration of _____