Hoping to reduce medication errors and contain health care costs, policy makers are promoting electronic prescribing through Medicare and Medicaid financial incentives. Many e-prescribing systems provide electronic access to important information--for example, medications prescribed by physicians in other practices, patient formularies and generic alternatives--when physicians are deciding what medications to prescribe. However, physician practices with e-prescribing face challenges using these features effectively, according to a new qualitative study by the Center for Studying Health System Change (HSC) funded by the Agency for Healthcare Research and Quality (AHRQ). While most of the 24 practices studied reported that physicians had access to patient formulary information, only slightly more than half reported physician access to patient medication histories, and many physicians did not routinely review these sources of information when making prescribing decisions. Study respondents highlighted two barriers to use: (1) tools to view and import the data into patient records were cumbersome to use in some systems; and (2) the data were not always perceived as useful enough to warrant the additional time to access and review them, particularly during time-pressed patient visits. To support generic prescribing, practices typically set their system defaults to permit pharmacist substitution of generics; many practices also used other tools to more proactively identify and select generic alternatives at the point of prescribing. Overall, physicians who more strongly perceived the need for third-party data, those in practices with greater access to complete and accurate data, and those with easier-to-use e-prescribing systems were more likely to use these features consistently.