Number of medications and adverse drug events by unintentional poisoning among older adults in consideration of inappropriate drug use: a Swedish population-based matched case-control study

PURPOSE: This national, population-based study aims to determine the association between the number of prescribed medications and adverse drug events (ADE) by unintentional poisoning and examine this risk when known indicators of inappropriate drug use (IDU) are accounted for. METHODS: We employed a matched case-control design among people living in Sweden who were 50 years and older. Cases experiencing an ADE by unintentional poisoning resulting in hospitalization or death (n = 5336) were extracted from the National Health and Death Registers from January 2006 to December 2009. Four controls per case matched by age, sex and residential area were randomly selected among those without an ADE (n = 21,344). Prescribed medications dispensed during the 4-month period prior to the ADE were identified via the Swedish Prescribed Drug Register and coded according to the number of different dispensed medications (NDDM) (0 to 10 medications) and IDU indicators (one single-drug, and three drug-combinations). Conditional logistic regression was used. RESULTS: Each of the IDU indicators was significantly associated with very high risks of ADE. For NDDM, we found a lower but graded positive association from two to ten or more medications (adjusted OR, 1.5; 95% CI, 1.2–1.8). Exclusion of IDU from the NDDM decreased the risk of ADE, but the effects remained significant for three or more medications (adjusted OR excl. IDU, 1.5; 95% CI, 1.2–2.0). CONCLUSION: At population level, the number of different dispensed medications starting from three or more remains associated with ADE even after adjusting for known IDUs. Clinicians and patients need to be made aware of the increased likelihood of serious ADE, not only in case of documented inappropriate medications but also in the case of an increasing number of medications. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s00228-017-2220-8) contains supplementary material, which is available to authorized users.