I don't have a link available, but I will try to describe it.
Insurance company A needs to help with a wide variety of prescription needs -- cholesterol management, heart disease, kidney disease, cancers, MS, diabetes. Insurance company A negotiates with pharmaceutical companies B, C, and D for how much A will pay for any drug and what copay A will charge to the insured patient.
Pharma B makes a deal to ensure that their diabetes drugs are on tier 1 or preferred status (most likely to be approved/used/paid) in exchange for a very good deal to the insurance companies on the price for each prescription and a lower tier for their cholesterol drug. The agreement also states that all other diabetes drugs (insulin, let's say) will be tier 3 or need prior authorization. This is a contract that might last a year or 5 and is not public.
Company C makes a deal to get their MS drug as the only reasonably priced option in exchange for basically giving away their foot fungus drug. The insurance company is happy because every foot fungus prescription is pure profit. Company C is happy because they are they only game in town for MS treatments on that insurance provider.
Pharma D wants to get their insulin at a higher tier so that patients can afford it and not buy out of pocket. But Pharma B already has an agreement that all other insulins will be tier 3 or lower, so Pharma D is locked out of that insurance company.
The current health insurance and prescription system in the US __heavily__ favors large pharma companies that can negotiate on multiple grounds and leverage competing health interests. Small pharmas that offer a singular stellar product have a tremendous fight to even get past prior authorization.
