There are several mechanism by insurers for similar drugs. One is preferred drug list. For drugs not on preferred list, either they are not covered at all or patients have to pay high co-pay/co-insurance much more than the 20% VRTX and GILD are helping to pay. One example is Victoza, recently taken out of preferred drug list by some insurers due to price. Another is formulary list. Victrelis for example is on VA formulary, its usage has not declined like Incivek.
With drug price soaring, insurers are going to utilize these measures much more frequently than in the past.