It's not a question of not being allowed to produce it, it's anti-competitive practices by the pharmaceuticals industry, which capitalism rewards.

Specifically, drug manufacturers have repeatedly made lots of little changes to their existing insulin products in order to apply for new patents on them. This process, called “evergreening,” has discouraged competitors from developing new versions of existing insulins because they’d have to chase so many changes. This has slowed down innovation, along with “pay for delay” deals, in which insulin manufacturers pay competitors to not copy specific drugs for a period of time.

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Even though there are very few insulin products that have patent protection on the compound itself, the vast majority of insulin products still have patent protection on the pens and other devices that deliver the dose of insulin. Novo Nordisk has patents for Novolog, Novolin, and FIASP products; Sanofi has patents on the devices for all of its products; and Eli Lilly still has patents on some devices that deliver Humulin and Humalog.

The patent protection on the devices is significant. Because the pens and other insulin delivery devices can only be used on with one brand of insulin, competition on those products is effectively delayed. While a prospective competitor could develop a follow-on biologic or biosimilar of the insulin, it would have to develop its own delivery device.

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