Forecasting drug sales is a tricky business; just ask any biopharma CEO who has overpromised and underdelivered. The people who make it their business know that very well, and they update their numbers with every big shift in the market and every surprise result from a key clinical trial.

These days, the variables are many, with some more variable than others. Biosimilars are coming in the U.S., with several big products targeted in the next few years. We know that’s going to hit the original biologics; we aren’t sure how much. Payers are increasingly pressing for discounts, particularly in highly competitive fields like hepatitis C drugs, respiratory meds and diabetes treatments. How much will those discounts grow, and how much might the pressure spread beyond those areas? How might new approaches to cancer-drug reimbursement–such as Express Scripts’ ($ESRX) indication-specific project–affect sales of those meds?

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