A 39-year-old woman is evaluated for worsening headaches. Headache episodes initially developed while she was in high school but have become increasingly severe and frequent over the past 3 years. She describes these recent headaches as an intense, hemicranial, throbbing pain that occurs two or three times per week and is associated with nausea and photophobia. For the past year, she has experienced additional daily episodes of dull, mild, global head pressure without associated features. The patient also has asthma and mild depression. She stopped taking over-the-counter analgesics 6 months ago when they became ineffective; other medications are albuterol and fluticasone.

On physical examination, blood pressure is 122/78 and pulse rate is 74/min; BMI is 22. Other physical examination findings, including those from a neurologic examination, are unremarkable.

An MRI of the brain is normal.

Which of the following is the most appropriate treatment?