Exposure definitions can include simple dichotomous variables (e.g., ever vs. never exposed) or be more granular, including estimates of duration, exposure windows (e.g., current vs. past exposure) also referred to as risk periods, or dosage (e.g., current dosage, cumulative dosage over time). Consideration should be given to both the requirements of the study design and the availability of variables. Assumptions made when preparing drug exposure data for analysis have an impact on results: an unreported step in pharmacoepidemiological studies (Pharmacoepidemiol Drug Saf. 2018;27(7):781-8) demonstrates the effect of certain exposure assumptions on findings and provides a framework to report preparation of exposure data. The Methodology chapter of the book Drug Utilization Research. Methods and Applications (M. Elseviers, B. Wettermark, A.B. Almarsdottir et al. Ed. Wiley Blackwell, 2016) discusses different methods for data collection on drug utilisation.
The population included in these data sources follows a process of attrition: drugs that are prescribed are not necessarily dispensed, and drugs that are dispensed are not necessarily ingested. In Primary non-adherence in general practice: a Danish register study (Eur J Clin Pharmacol 2014;70(6):757-63), 9.3% of all prescriptions for new therapies were never redeemed at the pharmacy, with different percentages per therapeutic and patient groups. The attrition from dispensing to ingestion is even more difficult to measure, as it is compounded by uncertainties about which dispensed drugs are actually taken by the patients and the patients’ ability to provide an accurate account of their intake.