After identifying all patients who were recommended endocrine therapy, data will be linked to the Norwegian Prescription Database (NorPD). Adherence will be estimated based on to which extent the patients actually have effectuated their prescriptions at Norwegian pharmacies. In the 5-year long treatment period, the patients receive a prescription refill of tamoxifen or aromatase inhibitor tablets for a 100 days interval. Medication Possession Ratio (MPR) will be used as proxy for adherence. The estimations will be performed in the following way: For each patient, we will receive the total number of pills and the total length of time from first to latest prescription effectuated. MPR is the number of pills divided by the total number of days in the time from first prescription to discontinuation. Discontinuation will be estimated to date for latest effectuation of the prescription plus 100 days (due to the number of pills prescribed). From these estimations, we will categorize the patients in groups taking MPR and total period of time into consideration. Taking prescript medication equal to or more than 80% is considered as adherent (MPR ≥ 80 %). Likewise, non-adherence is defined as MPR \< 80 %.
