Labels, guidelines and payer policies often specify where a treatment belongs in the sequence. A medicine may be approved as first-line therapy for some patients and only after another treatment for others. Formulary rules such as step therapy effectively push a product to a later line in a given plan.
Line of therapy is central to how pharma teams understand their market. The number of patients reaching each line, how long they stay, and why they move on shape the forecast and the competitive picture. In oncology, where treatment sequences are well defined, it is often the main way the market is segmented.
Measuring it from data is harder than it sounds. Real-world prescription and claims data do not record lines directly, so analysts use rules: what counts as a new regimen, how long a gap ends a line, whether adding a drug starts a new line or continues the old one. Different rules give different answers, which is a common source of KPI drift between teams.
Writing those rules down, and using the same ones everywhere, is the only reliable way to compare line-of-therapy figures across reports.