Objectives: To characterize the types of patient-related errors that lead to adverse drug events (ADEs) and identify patients at high risk of such errors.

Design: A subanalysis within a cohort study of Medicare enrollees.

Setting: A large multispecialty group practice.

Participants: Thirty thousand Medicare enrollees followed over a 12-month period.

Measurements: Primary outcomes were ADEs, defined as injuries due to a medication, and potential ADEs, defined as medication errors with the potential to cause an injury. The subset of these events that were related to patient errors was identified.

Results: The majority of patient errors leading to adverse events (n=129) occurred in administering the medication (31.8%), modifying the medication regimen (41.9%), or not following clinical advice about medication use (21.7%). Patient-related errors most often involved hypoglycemic medications (28.7%), cardiovascular medications (21.7%), anticoagulants (18.6%), or diuretics (10.1%). Patients with medication errors did not differ from a comparison group in age or sex but were taking more regularly scheduled medications (compared with 0-2 medications, odds ratio (OR) for 3-4 medications=2.0, 95% confidence interval (CI)=0.9-4.2; OR for 5-6 medications=3.1, 95% CI=1.5-7.0; OR for >or=7 medications=3.3, 95% CI=1.5-7.0). The strongest association was with the Charlson Comorbidity Index (compared with a score of 0, OR for a score of 1-2=3.8, 95% CI=2.1-7.0; OR for a score of 3-4=8.6, 95% CI=4.3-17.0; OR for a score of >or=5=15.0, 95% CI=6.5-34.5).

Conclusion: The medication regimens of older adults present a range of difficulties with the potential for harm. Strategies are needed that specifically address the management of complex drug regimens.