Clinical Decision Support Systems consists of reminders and alerts that prompt health IT users either to do something they might otherwise forget or to avoid an action that might harm a patient. In the second category are drug interaction checkers included in the electronic prescribing modules of EHRs and standalone e-prescribing applications. These systems warn physicians when a drug they are prescribing might have an adverse interaction with another medication the patient is taking or with a known allergy of the patient. Many EHRs also have health maintenance alerts that remind physicians to provide preventive or chronic care services that are recommended for patients with particular characteristics. These are not as robust as standalone electronic registries, which contain more specific and timely information about patients. Meaningful use requires use of CDS.