Insulin prices are soaring, creating pain for patients whose lives depend on the injectable drug—yet most of the revenue from the increases isn’t going to the drug manufacturers. It is largely the middlemen that benefit.

The major manufacturers of insulin—Eli Lilly & Co. of Indianapolis, Novo Nordisk A/S of Denmark and Sanofi SA of France—are collecting about the same or less than they did several years ago. The price increases—top-selling insulins have more than doubled in price since 2011—reflect the growing role of middlemen...