http://www.medscape.com/viewarticle/841669#vp_1
"The incremental innovation has repeatedly precluded the formation of a generic insulin industry in North America when earlier patents expired. The history of insulin hasn't followed the standard chronology of pharmaceutical innovation, in which patent monopolies predictably give way to generic competition."
Indeed, "insulin is not a single entity but a family of related products that has evolved through incremental improvements. Subsequent iterations of insulin represented actual innovations, each one being safer, more effective, or more convenient than its predecessor."
Because of that, "generic drug manufacturers are unlikely to invest in producing older versions of insulin that may already be obsolete," Drs Greene and Riggs observe.
And this is the case despite the fact that studies don't consistently show that analogs produce better outcomes than do the older NPH and regular insulins, and the advantages should be weighed against the costs, particularly for patients without adequate health insurance.
