A medical error is an unexpected and undesired event (adverse event) that results directly from the health care or services you receive but which could have been prevented.

Errors may happen when a health professional fails to complete a planned action or treatment as it was intended, or uses an incorrect plan for an action or treatment, such as giving a patient the wrong medicine or dose or giving someone a medicine they are allergic to. Errors can also happen when medical equipment or devices are used incorrectly. Errors may also happen when medical equipment or devices fail. Adverse events, whether due to a medical error or not, are not complications of disease or expected side effects of medicines or other treatments. An adverse event may result in injury or death.