Incidence in pharmacoepidemiology: A conceptual framework for incidence of a single substance or group of substances with statins as an example

A framework for analysing incidence in pharmacoepidemiology and drug statistics is suggested using statins as an example. A new case of statin use (first‐ever use or recurrence of treatment) can be defined as new on the group (NoG), new on substance whether new on the group or not (NoS), new on substance and new on the group (NoS_and_NoG), new on substance and not new on the group (NoS_not_NoG). Method Individual‐level dispensations of statins 2006–2019 for 1 017 058 individuals with at least one dispensation 2019 in Sweden. Results With 12‐month run‐in, corresponding to at least 8 months without treatment, the incidence proportion of NoG was 13.39 new cases per 1000 inhabitants and 8.40 with 10‐year run‐in. Thus, 37% had first been treated with any statin between 12 months and 10 years before the index date. For atorvastatin, NoS was 10.69, NoS_and_NoG 9.99, and NoS_not_NoG 0.70 per 1000 inhabitants. 0.70 per 1000 inhabitants or 6.6% of new cases of atorvastatin represented a change from another statin during the run‐in. Conclusion It is essential to separate new cases that are new both on the substance and on the group from those that represent a change of therapy during the run‐in.