A medical error is the failure of a planned action to be completed as intended, or the use of a wrong plan to achieve a goal. That definition, originally developed by psychologist James Reason and adopted by the Institute of Medicine, captures two distinct problems: errors of execution (doing the right thing incorrectly) and errors of planning (doing the wrong thing altogether). Both mental misjudgments and physical or technical failures count. In practical terms, a medical error is any act of omission or commission in planning or execution that contributes, or could contribute, to an unintended result for a patient.
How Medical Errors Differ From Adverse Events
Not every bad outcome in healthcare is a medical error, and not every medical error causes harm. These distinctions matter because they shape how errors are tracked and prevented. An adverse event is any injury caused by medical management rather than by the patient’s underlying condition. Some adverse events are not preventable. During a standard, properly performed cardiac procedure, for example, a patient might still suffer a stroke from a blood clot. No one made a mistake, but the patient was harmed.
A preventable adverse event, by contrast, is one that resulted from care falling below accepted standards or from a failure to apply a known prevention strategy. A physician who performs only a limited exam on a patient with ongoing rectal bleeding, reassures the patient, and misses a curable colon cancer until it has spread to the liver has committed a preventable adverse event due to substandard care. The line between “unfortunate outcome” and “error” often comes down to whether the care met the standards that a reasonable clinician in the same community would follow.
There is also a category called ameliorable adverse events. These are situations where the harm itself wasn’t preventable, but its severity could have been significantly reduced if different steps had been taken.
Near Misses: Errors That Don’t Reach the Patient
A near miss is an error that could have caused harm but didn’t, either because someone caught it in time or because of sheer luck. The World Health Organization defines it as “an error that has the potential to cause an adverse event but fails to do so because of chance or because it is intercepted.” A pharmacist catching a dangerous drug interaction before filling a prescription is a near miss. So is a nurse noticing the wrong medication bag seconds before connecting it to an IV line.
Near misses are enormously valuable for improving safety precisely because they share the same root causes as errors that do reach patients. They’re also far more common, which gives hospitals a much larger pool of data to analyze. And because no one was harmed, staff are more willing to report them without fear of blame or legal consequences. Some organizations even reward employees for flagging near misses, since each report reveals a weak point in the system that can be fixed before someone gets hurt.
How Common Are Medical Errors?
Medical errors are now recognized as the third leading cause of death in the United States. Estimates suggest that more than 200,000 patient deaths each year result from preventable medical errors, and roughly 400,000 hospitalized patients experience some form of preventable harm annually. Diagnostic errors alone cause the death or serious injury of 40,000 to 80,000 patients per year, according to the Joint Commission. One study estimated that 12 million Americans receive an incorrect diagnosis during their care each year, with about a third of those errors resulting in injury.
A systematic review pooling data from over 80,000 hospitalized adults found that at least 0.7% of adult admissions involve a harmful diagnostic error. That number may sound small, but applied across the millions of hospital admissions each year, it translates into an enormous volume of preventable harm. The economic burden is similarly staggering: one actuarial analysis estimated the annual cost of measurable medical errors at $17.1 billion, driven largely by post-surgical complications, healthcare-associated infections, and pressure injuries. A subset of particularly egregious errors, sometimes called “never events” (wrong-site surgery, retained surgical instruments, and similar failures that should never happen), accounted for roughly $3.7 billion of that total.
Common Types of Medical Errors
Medication errors are among the most frequent. They include giving the wrong drug, the wrong dose, or the wrong route of administration, as well as giving the right medication to the wrong patient. Nearly half of all medication errors occur at the prescribing or ordering stage, before a pharmacist or nurse ever touches the drug. Common prescribing failures include inaccurate order transcription, failing to check a patient’s allergy history, incomplete order review, and poor communication between team members.
Pharmacists add another layer of risk. Their errors tend to be either judgmental (missing a drug interaction, failing to counsel the patient, inadequate monitoring) or mechanical (dispensing the wrong strength or quantity). Factors that drive pharmacy errors include heavy workload, similar-sounding drug names, frequent interruptions, insufficient staffing, and illegible handwriting on prescriptions.
Diagnostic errors form the other major category. These range from delayed diagnoses to outright misdiagnoses. A diagnosis can go wrong at any point: a doctor might not order the right test, a lab might process a sample incorrectly, a radiologist might miss a finding on a scan, or a result might sit in a patient’s chart without anyone acting on it.
Surgical errors, hospital-acquired infections, communication breakdowns during handoffs between providers, and failures in follow-up care round out the landscape. Many of these overlap. A surgical complication might trace back to a communication failure in the operating room, which itself was enabled by a flawed scheduling system that left the team fatigued.
Why Errors Happen: The Swiss Cheese Model
The most widely used framework for understanding medical errors is the Swiss Cheese Model, based on James Reason’s theory of active and latent failures. It treats a healthcare system as a series of defensive layers, like slices of Swiss cheese. Each layer has gaps, represented by the holes in the cheese. Most of the time, one layer’s strengths cover another layer’s weaknesses. An error happens when the holes across multiple layers line up at the same time, allowing a hazard to pass through every defense and reach the patient.
Reason identified four levels where failures occur: organizational influences (budget decisions, staffing policies, safety culture), supervisory factors (inadequate training, poor oversight), preconditions for unsafe acts (fatigue, equipment malfunctions, communication breakdowns), and the unsafe acts themselves (a clinician selecting the wrong medication or misreading a chart). The key insight is that most patient harm events involve multiple failures across several of these levels simultaneously. Blaming a single nurse or doctor for an error almost always misses the broader system failures that made the error possible.
How Healthcare Organizations Respond
The traditional response to medical errors was to identify and punish the individual who made the mistake. This approach discouraged reporting and left the underlying system weaknesses untouched. Modern patient safety thinking has shifted toward what’s known as a Just Culture, which tries to balance openness with accountability.
In a Just Culture, staff are encouraged to report mistakes and near misses honestly, without fear of automatic punishment. The focus shifts from “who made the error” to “why did the system allow this to happen.” When someone makes an honest mistake (believing their action was correct), the appropriate response is coaching and system redesign, not discipline. When someone consciously drifts from safe practices, that’s treated as at-risk behavior requiring intervention and correction. Reckless behavior, where someone knowingly disregards a substantial and unjustifiable risk, can warrant disciplinary action or even criminal charges.
This framework matters because the vast majority of medical errors stem from honest mistakes made by competent people working within flawed systems. Punishing those individuals drives error reporting underground. A hospital that doesn’t know about its near misses and small failures can’t fix the system weaknesses that will eventually produce a catastrophic one. The goal of a reporting system is not to count errors but to identify their root causes, correct them, and prevent the next one.