Whether you are forecasting the uptake of a new cell and gene therapy or its impact on existing chronic therapies, there are two major headwinds. First, one-off treatments such as cell and gene therapies eliminate their own treatable patient pool. Second, the uncertainty of a cell and gene therapy’s long-term effectiveness. In this paper, we unveil how to effectively overcome these forecasting challenges and reveal how existing chronic therapies may, in time, get a “second bite of the apple”.