Prophylaxis of migraine headache.

A 40-year-old woman presented with an eight-year history of migraine headaches. The headaches occurred once or twice per week, but usually responded well to a triptan taken soon after onset. Sometimes the headache returned the next day, necessitating a second dose. Occasionally the headaches did not respond well to the triptan; they caused the patient to miss work about once per month. The patient’s headache diary shows that she takes a triptan nine days per month. Her physician suggests a prophylactic medication, given that she is at risk for medication overuse headache and experiencing substantial disability despite taking the medication. The patient’s medical history includes asthma that is well controlled. She generally sleeps well and has no history of clinically significant mood disturbance. Physical examination is normal except for obesity. Which prophylactic medication should be tried first?