What works in medication reconciliation: an on-treatment and site analysis of the MARQUIS2 study.

BACKGROUND: The second Multicenter Medication Reconciliation Quality Improvement Study demonstrated a marked reduction in medication discrepancies per patient. The aim of the current analysis was to determine the association of patient exposure to each system-level intervention and receipt of each patient-level intervention on these results. METHODS: This study was conducted at 17 North American Hospitals, the study period was 18 months per site, and sites typically adopted interventions after 2-5 months of preintervention data collection. We conducted an on-treatment analysis (ie, an evaluation of outcomes based on patient exposure) of system-level interventions, both at the category level and at the individual component level, based on monthly surveys of implementation site leads at each site (response rate 65%). We then conducted a similar analysis of patient-level interventions, as determined by study pharmacist review of documented activities in the medical record. We analysed the association of each intervention on the adjusted number of medication discrepancies per patient in admission and discharge orders, based on a random sample of up to 22 patients per month per site, using mixed-effects Poisson regression with hospital site as a random effect. We then used a generalised linear mixed-effects model (GLMM) decision tree to determine which patient-level interventions explained the most variance in discrepancy rates. RESULTS: Among 4947 patients, patient exposure to seven of the eight system-level component categories was associated with modest but significant reductions in discrepancy rates (adjusted rate ratios (ARR) 0.75-0.97), as were 15 of the 17 individual system-level intervention components, including hiring, reallocating and training personnel to take a best possible medication history (BPMH) and training personnel to perform discharge medication reconciliation and patient counselling. Receipt of five of seven patient-level interventions was independently associated with large reductions in discrepancy rates, including receipt of a BPMH in the emergency department (ED) by a trained clinician (ARR 0.40, 95% CI 0.37 to 0.43), admission medication reconciliation by a trained clinician (ARR 0.57, 95% CI 0.50 to 0.64) and discharge medication reconciliation by a trained clinician (ARR 0.64, 95% CI 0.57 to 0.73). In GLMM decision tree analyses, patients who received both a BPMH in the ED and discharge medication reconciliation by a trained clinician experienced the lowest discrepancy rates (0.08 per medication per patient). CONCLUSION AND RELEVANCE: Patient-level interventions most associated with reductions in discrepancies were receipt of a BPMH of admitted patients in the ED and admission and discharge medication reconciliation by a trained clinician. System-level interventions were associated with modest reduction in discrepancies for the average patient but are likely important to support patient-level interventions and may reach more patients. These findings can be used to help hospitals and health systems prioritise interventions to improve medication safety during care transitions.

Effects of a refined evidence-based toolkit and mentored implementation on medication reconciliation at 18 hospitals: results of the MARQUIS2 study.

BACKGROUND: The first Multicenter Medication Reconciliation Quality Improvement (QI) Study (MARQUIS1) demonstrated that mentored implementation of a medication reconciliation best practices toolkit decreased total unintentional medication discrepancies in five hospitals, but results varied by site. The objective of this study was to determine the effects of a refined toolkit on a larger group of hospitals. METHODS: We conducted a pragmatic quality improvement study (MARQUIS2) at 18 North American hospitals or hospital systems from 2016 to 2018. Incorporating lessons learnt from MARQUIS1, we implemented a refined toolkit, offering 17 system-level and 6 patient-level interventions. One of eight physician mentors coached each site via monthly calls and performed one to two site visits. The primary outcome was number of unintentional medication discrepancies in admission or discharge orders per patient. Time series analysis used multivariable Poisson regression. RESULTS: A total of 4947 patients were sampled, including 1229 patients preimplementation and 3718 patients postimplementation. Both the number of system-level interventions adopted per site and the proportion of patients receiving patient-level interventions increased over time. During the intervention, patients experienced a steady decline in their medication discrepancy rate from 2.85 discrepancies per patient to 0.98 discrepancies per patient. An interrupted time series analysis of the 17 sites with sufficient data for analysis showed the intervention was associated with a 5% relative decrease in discrepancies per month over baseline temporal trends (adjusted incidence rate ratio: 0.95, 95% CI 0.93 to 0.97, p<0.001). Receipt of patient-level interventions was associated with decreased discrepancy rates, and these associations increased over time as sites adopted more system-level interventions. CONCLUSION: A multicentre medication reconciliation QI initiative using mentored implementation of a refined best practices toolkit, including patient-level and system-level interventions, was associated with a substantial decrease in unintentional medication discrepancies over time. Future efforts should focus on sustainability and spread.

The Effects of a Multifaceted Intervention to Improve Care Transitions Within an Accountable Care Organization: Results of a Stepped-Wedge Cluster-Randomized Trial.

BACKGROUND: Transitions from hospital to the ambulatory setting are high risk for patients in terms of adverse events, poor clinical outcomes, and readmission. OBJECTIVES: To develop, implement, and refine a multifaceted care transitions intervention and evaluate its effects on postdischarge adverse events. DESIGN, SETTING, AND PARTICIPANTS: Two-arm, single-blind (blinded outcomes assessor), stepped-wedge, cluster-randomized clinical trial. Participants were 1,679 adult patients who belonged to one of 17 primary care practices and were admitted to a medical or surgical service at either of two participating hospitals within a pioneer accountable care organization (ACO). INTERVENTIONS: Multicomponent intervention in the 30 days following hospitalization, including inpatient pharmacist-led medication reconciliation, coordination of care between an inpatient "discharge advocate" and a primary care "responsible outpatient clinician," postdischarge phone calls, and postdischarge primary care visit. MAIN OUTCOMES AND MEASURES: The primary outcome was rate of postdischarge adverse events, as assessed by a 30-day postdischarge phone call and medical record review and adjudicated by two blinded physician reviewers. Secondary outcomes included preventable adverse events, new or worsening symptoms after discharge, and 30-day nonelective hospital readmission. RESULTS: Among patients included in the study, 692 were assigned to usual care and 987 to the intervention. Patients in the intervention arm had a 45% relative reduction in postdischarge adverse events (18 vs 23 events per 100 patients; adjusted incidence rate ratio, 0.55; 95% CI, 0.35-0.84). Significant reductions were also seen in preventable adverse events and in new or worsening symptoms, but there was no difference in readmission rates. CONCLUSION: A multifaceted intervention was associated with a significant reduction in postdischarge adverse events but no difference in 30-day readmission rates.

Effects of an Intensive Discharge Intervention on Medication Adherence, Glycemic Control, and Readmission Rates in Patients With Type 2 Diabetes.

OBJECTIVES: Patients with diabetes are at particularly high risk for adverse outcomes after hospitalization. The goals of this study were to design, implement, and evaluate a multipronged transitional care intervention among hospitalized patients with diabetes. METHODS: We randomly assigned inpatients likely to be discharged home on insulin to an intensive transitional care intervention or usual care. The primary outcome was 90-day postdischarge insulin adherence, using prescription refill information to calculate a medication possession ratio. Unadjusted analyses were conducted using Wilcoxon rank sum; adjusted analyses used multivariable linear regression and weighted propensity scoring methods, with general estimating equations to account for clustering by admitting physician. RESULTS: One hundred eighty patients participated. The mean (SD) medication possession ratio to all insulin types was 84.5% (22.6) among intervention and 76.4% (25.1) among usual care patients (difference = 8.1, 95% confidence interval = -1.0 to 17.2, P = 0.06), with a smaller difference for adherence to all medications (86.3% versus 82.0%). A1c levels decreased in both groups but was larger in the intervention arm (1.09 and 0.11, respectively) (difference = -0.98, 95% confidence interval = -2.03 to -0.07, P = 0.04). Differences between study arms were not significant for rates of hypoglycemic episodes, 30-day readmissions, or emergency department visits. In adjusted/clustered analyses, the difference in A1c reduction remained statistically significant, whereas differences in all other outcomes remained nonsignificant. CONCLUSIONS: The intervention was associated with improvements in glycemic control, with nonsignificant trends toward greater medication adherence. Further research is needed to optimize and successfully implement interventions to improve patient safety and health outcomes during care transitions.

Design of MARQUIS2: study protocol for a mentored implementation study of an evidence-based toolkit to improve patient safety through medication reconciliation.

BACKGROUND: The first Multi-center Medication Reconciliation Quality Improvement Study (MARQUIS1) demonstrated that implementation of a medication reconciliation best practices toolkit decreased total unintentional medication discrepancies in five hospitals. We sought to implement the MARQUIS toolkit in more diverse hospitals, incorporating lessons learned from MARQUIS1. METHODS: MARQUIS2 is a pragmatic, mentored implementation QI study which collected clinical and implementation outcomes. Sites implemented a revised toolkit, which included interventions from these domains: 1) best possible medication history (BPMH)-taking; 2) discharge medication reconciliation and patient/caregiver counseling; 3) identifying and defining clinician roles and responsibilities; 4) risk stratification; 5) health information technology improvements; 6) improved access to medication sources; 7) identification and correction of real-time discrepancies; and, 8) stakeholder engagement. Eight hospitalists mentored the sites via one site visit and monthly phone calls over the 18-month intervention period. Each site's local QI team assessed opportunities to improve, implemented at least one of the 17 toolkit components, and accessed a variety of resources (e.g. implementation manual, webinars, and workshops). Outcomes to be assessed will include unintentional medication discrepancies per patient. DISCUSSION: A mentored multi-center medication reconciliation QI initiative using a best practices toolkit was successfully implemented across 18 medical centers. The 18 participating sites varied in size, teaching status, location, and electronic health record (EHR) platform. We introduce barriers to implementation and lessons learned from MARQUIS1, such as the importance of utilizing dedicated, trained medication history takers, simple EHR solutions, clarifying roles and responsibilities, and the input of patients and families when improving medication reconciliation.

An On-Treatment Analysis of the MARQUIS Study: Interventions to Improve Inpatient Medication Reconciliation.

It is unclear which medication reconciliation interventions are most effective at reducing inpatient medication discrepancies. Five United States hospitals' interdisciplinary quality improvement (QI) teams were virtually mentored by QI-trained physicians. Sites implemented one to seven evidence-based interventions in 791 patients during the 25-month implementation period. Three interventions were associated with significant decreases in potentially harmful discrepancy rates: (1) defining clinical roles and responsibilities, (2) training, and (3) hiring staff to perform discharge medication reconciliation. Two interventions were associated with significant increases in potentially harmful discrepancy rates: training staff to take medication histories and implementing a new electronic health record (EHR). Hospitals should focus first on hiring and training pharmacy staff to assist with medication reconciliation at discharge and delineating roles and responsibilities of clinical staff. We caution hospitals implementing a large vendor EHR, as medication discrepancies may increase. Finally, the effect of medication history training on discrepancies needs further study.

Effects of a multifaceted medication reconciliation quality improvement intervention on patient safety: final results of the MARQUIS study.

BACKGROUND: Unintentional discrepancies across care settings are a common form of medication error and can contribute to patient harm. Medication reconciliation can reduce discrepancies; however, effective implementation in real-world settings is challenging. METHODS: We conducted a pragmatic quality improvement (QI) study at five US hospitals, two of which included concurrent controls. The intervention consisted of local implementation of medication reconciliation best practices, utilising an evidence-based toolkit with 11 intervention components. Trained QI mentors conducted monthly site phone calls and two site visits during the intervention, which lasted from December 2011 through June 2014. The primary outcome was number of potentially harmful unintentional medication discrepancies per patient; secondary outcome was total discrepancies regardless of potential for harm. Time series analysis used multivariable Poisson regression. RESULTS: Across five sites, 1648 patients were sampled: 613 during baseline and 1035 during the implementation period. Overall, potentially harmful discrepancies did not decrease over time beyond baseline temporal trends, adjusted incidence rate ratio (IRR) 0.97 per month (95% CI 0.86 to 1.08), p=0.53. The intervention was associated with a reduction in total medication discrepancies, IRR 0.92 per month (95% CI 0.87 to 0.97), p=0.002. Of the four sites that implemented interventions, three had reductions in potentially harmful discrepancies. The fourth site, which implemented interventions and installed a new electronic health record (EHR), saw an increase in discrepancies, as did the fifth site, which did not implement any interventions but also installed a new EHR. CONCLUSIONS: Mentored implementation of a multifaceted medication reconciliation QI initiative was associated with a reduction in total, but not potentially harmful, medication discrepancies. The effect of EHR implementation on medication discrepancies warrants further study. TRIAL REGISTRATION NUMBER: NCT01337063.

A Systematic Review of Beliefs About Hypertension and its Treatment Among African Americans.

PURPOSE OF REVIEW: The prevalence of hypertension and uncontrolled hypertension is higher among African Americans than any other ethnicity in the USA. Certain patient medical beliefs may lead to adverse health behaviors. The aim of this study was to systematically review and narratively synthesize beliefs about hypertension among African Americans. RECENT FINDINGS: In a narrative review of 22 studies, many participants attributed hypertension to stress and fatty foods. Hypertension was perceived to be an episodic, symptomatic disease. Many patients exhibited a strong faith in the efficacy of medications, but used them as needed to treat perceived intermittent hypertensive episodes or infrequently to avoid addiction and dependence. Home remedies were often reported to be used concurrently to treat the folk disease "high blood" or in place of medications associated with unwanted effects. Nevertheless, participants were invested in treatment of hypertension to prevent long-term complications. Trends over time suggest that beliefs about hypertension among African Americans have change significantly and now reflect the currently accepted biomedical model. African American beliefs about hypertension may frequently differ from those of healthcare professionals. These results suggest that reconciliation of differences between patient and provider expectations for disease management may improve adherence to and acceptance of medical treatments among African Americans with hypertension. Nevertheless, discordant health beliefs are common among all patients and additional work to elucidate beliefs of other patient subgroups such as age and gender is warranted.

Effect of Pharmacist Counseling Intervention on Health Care Utilization Following Hospital Discharge: A Randomized Control Trial.

BACKGROUND: Reduction in 30-day readmission rates following hospitalization for acute coronary syndrome (ACS) and acute decompensated heart failure (ADHF) is a national goal. OBJECTIVE: The aim of this study was to determine the effect of a tailored, pharmacist-delivered, health literacy intervention on unplanned health care utilization, including hospital readmission or emergency room (ER) visit, following discharge. DESIGN: Randomized, controlled trial with concealed allocation and blinded outcome assessors SETTING: Two tertiary care academic medical centers PARTICIPANTS: Adults hospitalized with a diagnosis of ACS and/or ADHF. INTERVENTION: Pharmacist-assisted medication reconciliation, inpatient pharmacist counseling, low-literacy adherence aids, and individualized telephone follow-up after discharge MAIN MEASURES: The primary outcome was time to first unplanned health care event, defined as hospital readmission or an ER visit within 30 days of discharge. Pre-specified analyses were conducted to evaluate the effects of the intervention by academic site, health literacy status (inadequate versus adequate), and cognition (impaired versus not impaired). Adjusted hazard ratios (aHR) and 95% confidence intervals (CI) are reported. KEY RESULTS: A total of 851 participants enrolled in the study at Vanderbilt University Hospital (VUH) and Brigham and Women's Hospital (BWH). The primary analysis showed no statistically significant effect on time to first unplanned hospital readmission or ER visit among patients who received interventions compared to controls (aHR = 1.04, 95% CI 0.78-1.39). There was an interaction of treatment effect by site (p = 0.04 for interaction); VUH aHR = 0.77, 95% CI 0.51-1.15; BWH aHR = 1.44 (95% CI 0.95-2.12). The intervention reduced early unplanned health care utilization among patients with inadequate health literacy (aHR 0.41, 95% CI 0.17-1.00). There was no difference in treatment effect by patient cognition. CONCLUSION: A tailored, pharmacist-delivered health literacy-sensitive intervention did not reduce post-discharge unplanned health care utilization overall. The intervention was effective among patients with inadequate health literacy, suggesting that targeted practice of pharmacist intervention in this population may be advantageous.

Evaluation of a Pharmacist-Directed Vancomycin Dosing and Monitoring Pilot Program at a Tertiary Academic Medical Center.

BACKGROUND: Consensus guidelines recommend vancomycin doses of 15 to 20 mg/kg every 8 to 12 hours in patients with normal renal function. OBJECTIVE: To evaluate the effect of a pharmacist-directed vancomycin dosing and monitoring pilot program on the percentage of patients receiving targeted weight-based dosing recommendations. METHODS: This was a pre-/postevaluation study, approved by the institutional review board at our institution, comparing retrospectively reviewed vancomycin dosing practices hospital-wide between September 1 and September 30, 2010 to patients prospectively managed by a pharmacist-directed vancomycin pilot program between February 1 and April 26, 2011. All adult inpatients receiving intravenous vancomycin were included, unless patients had a creatinine clearance less than or equal to 60 mL/min or indication for therapy was surgical prophylaxis or febrile neutropenia. The primary outcome was the percentage of patients who received optimal vancomycin dosing defined as ≥30 mg/kg/d within 24 hours of initiation of therapy. Secondary outcomes included number of pharmacist interventions, length of therapy and incidence of nephrotoxicity while receiving vancomycin. RESULTS: A total of 319 patients were analyzed, 161 preimplementation and 158 postimplementation. The percentage of patients who received optimal vancomycin dosing was significantly higher postimplementation of the pilot program, 96.8 versus 40.4% (P < 0.001). Pharmacist-directed interventions postimplementation, resulted in 50% more patients being dosed optimally (P < 0.001). Patients in the pilot program also had a shorter length of therapy (10.0 vs 8.4 days, P < 0.003) and a lower incidence of nephrotoxicity (8.7% vs 3.2%, P = 0.006). CONCLUSIONS: This pharmacist-directed vancomycin pilot program significantly increased the percentage of patients optimally dosed according to consensus guidelines within 24 hours of initiation of therapy.

Rationale and design of the Multicenter Medication Reconciliation Quality Improvement Study (MARQUIS).

BACKGROUND: Unresolved medication discrepancies during hospitalization can contribute to adverse drug events, resulting in patient harm. Discrepancies can be reduced by performing medication reconciliation; however, effective implementation of medication reconciliation has proven to be challenging. The goals of the Multi-Center Medication Reconciliation Quality Improvement Study (MARQUIS) are to operationalize best practices for inpatient medication reconciliation, test their effect on potentially harmful unintentional medication discrepancies, and understand barriers and facilitators of successful implementation. METHODS: Six U.S. hospitals are participating in this quality improvement mentored implementation study. Each hospital has collected baseline data on the primary outcome: the number of potentially harmful unintentional medication discrepancies per patient, as determined by a trained on-site pharmacist taking a "gold standard" medication history. With the guidance of their mentors, each site has also begun to implement one or more of 11 best practices to improve medication reconciliation. To understand the effect of the implemented interventions on hospital staff and culture, we are performing mixed methods program evaluation including surveys, interviews, and focus groups of front line staff and hospital leaders. DISCUSSION: At baseline the number of unintentional medication discrepancies in admission and discharge orders per patient varies by site from 2.35 to 4.67 (mean=3.35). Most discrepancies are due to history errors (mean 2.12 per patient) as opposed to reconciliation errors (mean 1.23 per patient). Potentially harmful medication discrepancies averages 0.45 per patient and varies by site from 0.13 to 0.82 per patient. We discuss several barriers to implementation encountered thus far. In the end, we anticipate that MARQUIS tools and lessons learned have the potential to decrease medication discrepancies and improve patient outcomes. TRIAL REGISTRATION: Clinicaltrials.gov identifier NCT01337063.

Effect of patient- and medication-related factors on inpatient medication reconciliation errors.

BACKGROUND: Little research has examined the incidence, clinical relevance, and predictors of medication reconciliation errors at hospital admission and discharge. OBJECTIVE: To identify patient- and medication-related factors that contribute to pre-admission medication list (PAML) errors and admission order errors, and to test whether such errors persist in the discharge medication list. DESIGN, PARTICIPANTS: We conducted a cross-sectional analysis of 423 adults with acute coronary syndromes or acute decompensated heart failure admitted to two academic hospitals who received pharmacist-assisted medication reconciliation during the Pharmacist Intervention for Low Literacy in Cardiovascular Disease (PILL-CVD) Study. MAIN MEASURES: Pharmacists assessed the number of total and clinically relevant errors in the PAML and admission and discharge medication orders. We used negative binomial regression and report incidence rate ratios (IRR) of predictors of reconciliation errors. KEY RESULTS: On admission, 174 of 413 patients (42%) had ≥1 PAML error, and 73 (18%) had ≥1 clinically relevant PAML error. At discharge, 158 of 405 patients (39%) had ≥1 discharge medication error, and 126 (31%) had ≥1 clinically relevant discharge medication error. Clinically relevant PAML errors were associated with older age (IRR = 1.46; 95% CI, 1.00- 2.12) and number of pre-admission medications (IRR = 1.17; 95% CI, 1.10-1.25), and were less likely when a recent medication list was present in the electronic medical record (EMR) (IRR = 0.54; 95% CI, 0.30-0.96). Clinically relevant admission order errors were also associated with older age and number of pre-admission medications. Clinically relevant discharge medication errors were more likely for every PAML error (IRR = 1.31; 95% CI, 1.19-1.45) and number of medications changed prior to discharge (IRR = 1.06; 95% CI, 1.01-1.11). CONCLUSIONS: Medication reconciliation errors are common at hospital admission and discharge. Errors in preadmission medication histories are associated with older age and number of medications and lead to more discharge reconciliation errors. A recent medication list in the EMR is protective against medication reconciliation errors.

Rationale and design of the Pharmacist Intervention for Low Literacy in Cardiovascular Disease (PILL-CVD) study.

BACKGROUND: Medication errors and adverse drug events are common after hospital discharge due to changes in medication regimens, suboptimal discharge instructions, and prolonged time to follow-up. Pharmacist-based interventions may be effective in promoting the safe and effective use of medications, especially among high-risk patients such as those with low health literacy. METHODS AND RESULTS: The Pharmacist Intervention for Low Literacy in Cardiovascular Disease (PILL-CVD) study is a randomized controlled trial conducted at 2 academic centers-Vanderbilt University Hospital and Brigham and Women's Hospital. Patients admitted with acute coronary syndrome or acute decompensated heart failure were randomly assigned to usual care or intervention. The intervention consisted of pharmacist-assisted medication reconciliation, inpatient pharmacist counseling, low-literacy adherence aids, and tailored telephone follow-up after discharge. The primary outcome is the occurrence of serious medication errors in the first 30 days after hospital discharge. Secondary outcomes are health care utilization, disease-specific quality of life, and cost-effectiveness. Enrollment was completed September 2009. A total of 862 patients were enrolled, and 430 patients were randomly assigned to receive the intervention. Analyses will determine whether the intervention was effective in reducing serious medication errors, particularly in patients with low health literacy. CONCLUSIONS: The PILL-CVD study was designed to reduce serious medication errors after hospitalization through a pharmacist-based intervention. The intervention, if effective, will inform health care facilities on the use of pharmacist-assisted medication reconciliation, inpatient counseling, low-literacy adherence aids, and patient follow-up after discharge. Clinical Trial Registration- clinicaltrials.gov. Identifier: NCT00632021.

Effect of an electronic medication reconciliation application and process redesign on potential adverse drug events: a cluster-randomized trial.

BACKGROUND: Medication reconciliation at transitions in care is a national patient safety goal, but its effects on important patient outcomes require further evaluation. We sought to measure the impact of an information technology-based medication reconciliation intervention on medication discrepancies with potential for harm (potential adverse drug events [PADEs]). METHODS: We performed a controlled trial, randomized by medical team, on general medical inpatient units at 2 academic hospitals from May to June 2006. We enrolled 322 patients admitted to 14 medical teams, for whom a medication history could be obtained before discharge. The intervention was a computerized medication reconciliation tool and process redesign involving physicians, nurses, and pharmacists. The main outcome was unintentional discrepancies between preadmission medications and admission or discharge medications that had potential for harm (PADEs). RESULTS: Among 160 control patients, there were 230 PADEs (1.44 per patient), while among 162 intervention patients there were 170 PADEs (1.05 per patient) (adjusted relative risk [ARR], 0.72; 95% confidence interval [CI], 0.52-0.99). A significant benefit was found at hospital 1 (ARR, 0.60; 95% CI, 0.38-0.97) but not at hospital 2 (ARR, 0.87; 95% CI, 0.57-1.32) (P = .32 for test of effect modification). Hospitals differed in the extent of integration of the medication reconciliation tool into computerized provider order entry applications at discharge. CONCLUSIONS: A computerized medication reconciliation tool and process redesign were associated with a decrease in unintentional medication discrepancies with potential for patient harm. Software integration issues are likely important for successful implementation of computerized medication reconciliation tools.

Classifying and predicting errors of inpatient medication reconciliation.

BACKGROUND: Failure to reconcile medications across transitions in care is an important source of potential harm to patients. Little is known about the predictors of unintentional medication discrepancies and how, when, and where they occur. OBJECTIVE: To determine the reasons, timing, and predictors of potentially harmful medication discrepancies. DESIGN: Prospective observational study. PATIENTS: Admitted general medical patients. MEASUREMENTS: Study pharmacists took gold-standard medication histories and compared them with medical teams' medication histories, admission and discharge orders. Blinded teams of physicians adjudicated all unexplained discrepancies using a modification of an existing typology. The main outcome was the number of potentially harmful unintentional medication discrepancies per patient (potential adverse drug events or PADEs). RESULTS: Among 180 patients, 2066 medication discrepancies were identified, and 257 (12%) were unintentional and had potential for harm (1.4 per patient). Of these, 186 (72%) were due to errors taking the preadmission medication history, while 68 (26%) were due to errors reconciling the medication history with discharge orders. Most PADEs occurred at discharge (75%). In multivariable analyses, low patient understanding of preadmission medications, number of medication changes from preadmission to discharge, and medication history taken by an intern were associated with PADEs. CONCLUSIONS: Unintentional medication discrepancies are common and more often due to errors taking an accurate medication history than errors reconciling this history with patient orders. Focusing on accurate medication histories, on potential medication errors at discharge, and on identifying high-risk patients for more intensive interventions may improve medication safety during and after hospitalization.