Patient outcomes for the insulin aspart pens and the vials of generic insulin at Walmart are so different that many doctors refuse to prescribe the cheap stuff except as a very last resort.
There's no simply substituting. These are two different brands of a generic statin pill or something. Although it's all insulin, these are very different drugs. The dosages for the same insulin in the same patient can vary, and the calculations or charts for what to take when vary across types of insulins. The handling instructions vary. There's different patient training for a pen vs. a vial and syringe.
There is no substituting but also no easy comparison of how much worse the regular stuff is, since the population using R is often underinsured and unable to afford other elements of prevention and care.
That fixes the pen issue, but it's still not the same insulin most doctors prescribe and can't be easily substituted without medical consultation. Having it available inexpensively and without a prescription is a compromise, per Novo Nordisk themselves.
