NBRx is the metric most brand teams watch to see whether they are winning new patients. It is calculated from patient-level prescription data: a patient counts as new to brand when they fill a prescription for it and have no record of it during the lookback period.
It is often confused with two related measures. TRx counts all prescriptions dispensed, new and refills. NRx counts new prescription numbers, which includes new prescriptions for patients who have been on the drug for years, for example when a prescription with no refills left is renewed. Neither measures new patients the way NBRx does.
The main differences between NBRx definitions come from three choices: how long the lookback period is (six, twelve or twenty-four months are all common), whether a patient restarting after a gap counts as new, and whether new-to-market patients, switches and add-ons are reported together or separately. Two teams using different choices can report very different growth from the same data.
That makes NBRx a classic case of KPI drift. Our guide to commercial KPI definitions includes a worked example.