To follow up, your doctor needs to say other migraine treatments were tried and failed in order for insurance to cover it (and continue to do so every year). Also, they maker of it owns a pharmacy that can reliably refill it every month, so it is easiest to go with them.

Expensive as hell, but unlike triptans (most effective when you are having an aura before the headache, not very effective after), you can take it at any phase of the migraine.