Background Specific types of medications, but also drug-drug interactions are known to lead to adverse drug events (ADE) among older people. However, only small numbers of controlled epidemiological studies investigated ADE with particular focus on the prescription of inappropriate medications (inappropriate drug use – IDU) or the number of medications prescribed. This based study investigates the association between the number of prescribed medications and ADEs, while considering different indicators of inappropriate drug use (IDU).

Methods In this study we utilised a matched case-control design among Swedish residents 50 years and older. The National Inpatient Register and the Causes of Death Register were used to extract cases with and ADE by unintentional poisoning that resulted in a hospitalisation or death within January 2006 to December 2009 (n = 5,336). Each case was matched with four controls, based on age, sex and residential area. The controls were randomly selected among the individuals without an ADE (n = 21,344). Any medication prescribed during the four-month period prior to the ADE were selected by using the Swedish Prescribed Drug Register and were then categorised based on the total number of medications prescribed and on correspondence with an IDU indicator (known single-drug and drug combinations). Effects were estimated with odds ratios (OR) and 95% confidence intervals (CI:) by applying conditional logistic regression.

Results Each of the indicators for IDU was associated with a high risk of ADE among older people. We found a lower, yet still positive and graded association between the total number of medications prescribed and the risk for ADE, starting by two medications to ten or more. The risk of ADE remained increased when the total number of medications prescribed was at three to ten or more medications, even after exclusion of IDU indicators from the total number of medications prescribed.

Conclusions The total number of medications and IDU indicators are associated with ADE among older people. Although IDU indicators were positively associated with ADE, they lacked to fully explain the graded association with increasing numbers of medications prescribed. Physicians, but also patients need to be aware of this increased likelihood of serious ADE by low and increasing numbers of medications prescribed.