But a normal insulin injection elsewhere in the body wouldn't work either. For one thing, such an injection might cause an accidental crash in the volunteer's blood sugar. For another, it's difficult to get the hormone across the finicky blood-brain barrier. This barrier keeps unwanted substances away from our brains, and it poses a big challenge for large molecules like insulin. In fact, until the 1970s, scientists didn't even realize we had insulin floating around our heads. But by using a nasal spray, the researchers were able to bypass all these problems and send the insulin directly to the central nervous system through the olfactory nerve.