The mix of drugs used for estimating the cost and outcomes for the budget-impact analysis will likely change over the analysis time horizon even without the introduction of the new drug. This could be because of changes in treatment patterns or increasing uptake of recently approved drugs or patent expiration of current drugs. When a new drug is added to the formulary, the mix of drugs used for the budget-impact analysis will also change and depend upon the uptake of the new drug, whether it is added to currently used drugs or which of the currently used treatments it replaces. In this chapter, issues that may affect the treatment mix and methods for determining the treatment mix with and without the new drug are presented. Potential sources of data are also discussed.
