Rationale: Adverse drug reactions (ADRs) are common and often severe in ambulatory elders. However, little is known about how classically "geriatric1 features such as frailty impact ADR risk, and few clinically- useful tools exist to quantify the risk of ADRs in individual older persons. Research objectives / aims: Our aims are (1) to determine risk factors for adverse drug reactions in elders, with a particular focus on geriatric features including physical, cognitive, and social vulnerability;(2) to develop and validate a prediction tool to assess ADR risk in elders;and (3) to use clinician focus groups and interviews to refine and pilot test an intervention based on the ADR risk prediction index. At the conclusion of the Beeson Award period, we will submit a R01 grant proposal to test this intervention. Methods: We will assemble and follow a prospective cohort of 400 elders following discharge from two San Francisco hospitals. Through serial telephone interviews and chart review, we will use validated methods to identify and characterize adverse drug reactions and to collect information on a variety of potential risk factors. For Aim (3), we will conduct focus groups and structured cognitive interviews to refine and then pilot test an intervention based on the ADR risk index. Analyses: For Aim 1, we will use mixed-efforts Poisson regression to determine risk factors for ADRs. For Aim 2, we will use standard techniques to develop and validate a risk prediction index for ADRs. For Aim 3, we will use qualitative techniques to analyze focus group and interview transcripts for relevant content. Relevance to public health: Quantifying ADR risk in elders can improve clinical care by helping clinicians weigh the benefits vs. harms of drug therapy and by targeting high-risk elders for systems-based interventions to reduce ADRs and improve prescribing. The proposed research will obtain data to support a future intervention study whose goal is to reduce ADRs and improve prescribing for elders.