When patents expire on profitable brand-name prescription drugs, patients and their insurance companies usually both catch a break on the price as generic manufacturers move in with cheaper versions of the same drug. That's happening now with Lipitor, the most prescribed brand name drug in the U.S., but the usual rules on pricing may be broken.

Every day 3.5 million people take Lipitor to control cholesterol. Since its debut in 1997, Lipitor has lined the coffers of Pfizer to the tune of $81 billion. It’s the best-selling prescription drug ever, and its patent protection ended in November.

A few months ago we wrote about how pharmaceutical companies whose patents are expiring for prescription drugs sometimes go to great—and questionable—lengths to prevent other manufacturers from making a generic version that costs a lot less. So it seems counterintuitive that a member of Big Pharma would celebrate the end of a drug protection period by lowering its cost below that of a generic.