How lucrative are those new migraine drugs, really?

Analysts expect a new class of preventive migraine treatments to reach beyond $5 billion in annual sales. But early returns suggest the byzantine system that determines which drugs get covered in the U.S. will make reaching that mark an uphill battle.

Yesterday, the nation’s largest pharmacy benefits manager decided it would cover the migraine treatments made by Amgen and Eli Lilly but not a third, virtually identical one from Teva Pharmaceutical. That’s bad for Teva and good for its competitors, but, according to Reuters, Amgen and Lilly had to promise Express Scripts a money-back guarantee when their drugs don’t work, which comes on top of the undisclosed rebatesthey offered to get their feet in the door.

That all sets the stage for a bottom-ward race for all three companies. As other insurance plans determine which migraine therapies to cover, the manufactures will have to jockey for favor by offering discounts, a process that could make the whole endeavor a lot less lucrative than Wall Street once figured.