Methods: The PRE2DUP method is a novel approach based on mathematical modelling of personal drug
purchasing behaviors. The method uses a decision procedure that includes each person’s purchase history for each
ATC code, processed in a chronological order. The method constructs exposure time periods and estimates the
dose used during the period by considering the purchased amount in Defined Daily Doses (DDDs), which is
recorded in the prescription register database. This method takes account of stockpiling of drugs, personal
purchasing pattern; i.e., regularity of the purchases, and periods of hospital or nursing home care where drug use is
not recorded in the prescription register. The method can be applied to a variety of drug classes with different
doses and use patterns by controlling restriction parameters for each ATC class, or even each drug package. In the
presented example, the PRE2DUP method was applied to a register-based MEDALZ-2005 study cohort. All drug
purchases (3,793,085) recorded in the Finnish prescription register between 2002 and 2009 for persons with
Alzheimer’s disease (28,093) were included.