The Committee on Data Standards for Patient Safety wanted to make clear that the potentially avoidable results of an underlying disease or condition—for example, a recurrent myocardial infarction in a patient without a contraindication who was not given a beta-blocker (an error of omission)—should be considered an adverse event (IOM, 2004). The Committee on Identifying and Preventing Medication Errors discussed the adverse event definition given in the Patient Safety report and decided to adopt this definition. Further attempts to operationalize the definition of adverse event may well lead eventually to additional modifications of the definition.

Consistent with the above definitions:

A medication error is defined as any error occurring in the medication use process (Bates et al., 1995a).

An adverse drug event is defined as any injury due to medication (Bates et al., 1995b).

An injury includes physical harm (for example, rash), mental harm (for example, confusion), or loss of function (for example, inability to drive a car).

Medication errors and adverse drug events have multiple sources. They may be related to professional practice; health care products, procedures, and systems, including prescribing; order communication; product labeling, packaging, and nomenclature; compounding; dispensing; distribution; administration; education of the patient or health care professional; and monitoring of use.

Implicit in the definition of medication errors is that they are preventable. However, most medication errors do not cause harm. Some do cause harm and are either potential adverse drug events or preventable adverse drug events (see Figure 1-1), depending on whether an injury occurred (Gandhi et al., 2000). Potential adverse drug events are events in which an error occurred but did not cause injury (for example, the error was detected before the patient was affected, or the patient received a wrong dose but experienced no harm) (Gandhi et al., 2000).

Adverse drug events can be preventable (for example, a wrong dose leads to injury) or nonpreventable (for example, an allergic reaction occurs in a patient not known to be allergic) (see Figure 1-1). Nonpreventable adverse drug events are also often termed adverse drug reactions3 (Gandhi et al., 2000).


The field of pharmacoepidemiology defines the terms adverse drug event and adverse drug reaction differently (Strom, 2005). An adverse drug event (or adverse drug experience) is an untoward outcome that occurs during or following clinical use of a drug, whether preventable or not. An adverse drug reaction is an adverse drug event that is judged to be caused by the drug.

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