Unnecessary health care (overutilization, overuse, or overtreatment) is health care provided with a higher volume or cost than is appropriate. In the United States, where health care costs are the highest as a percentage of GDP, overuse was the predominant factor in its expense, accounting for about a third of its health care spending ($750 billion out of $2.6 trillion) in 2012.

Factors that drive overuse include paying health professionals more to do more (fee-for-service), defensive medicine to protect against litigiousness, and insulation from price sensitivity in instances where the consumer is not the payer—the patient receives goods and services but insurance pays for them (whether public insurance, private, or both). Such factors leave many actors in the system (doctors, patients, pharmaceutical companies, device manufacturers) with inadequate incentive to restrain health care prices or overuse. This drives payers, such as national health insurance systems or the U.S. Centers for Medicare and Medicaid Services, to focus on medical necessity as a condition for payment. However, the threshold between necessity and lack thereof can often be subjective.

Overtreatment, in the strict sense, may refer to unnecessary medical interventions, including treatment of a self-limited condition (overdiagnosis) or to extensive treatment for a condition that requires only limited treatment.

It is economically linked with overmedicalization.

A forerunner of the term was what Jack Wennberg called unwarranted variation, different rates of treatments based upon where people lived, not clinical rationale. He had discovered that in studies that began in 1967 and were published in the 1970s and the 1980s: "The basic premise – that medicine was driven by science and by physicians capable of making clinical decisions based on well-established fact and theory – was simply incompatible with the data we saw. It was immediately apparent that suppliers were more important in driving demand than had been previously realized."

In 2008, US bioethicist Ezekiel J. Emanuel and health economist Victor R. Fuchs defined unnecessary health care as "overutilization", health care provided with a higher volume or cost than is appropriate. Recently, economists have sought to understand unnecessary health care in terms of misconsumption rather than overconsumption.

In 2009 two US physicians wrote in an editorial, that unnecessary care was "defined as services which show no demonstrable benefit to patients" and might represent 30% of U.S. medical care. They referred to a 2003 study on regional variations in Medicare spending, which found, "Medicare enrollees in higher-spending regions receive more care than those in lower-spending regions, but do not have better health outcomes or satisfaction with care."

In January 2012, the American College of Physicians Ethics, Professionalism, and Human Rights Committee suggested that overtreatment can also be understood in contrast to 'parsimonious care', defined as "care that utilizes the most efficient means to effectively diagnose a condition and treat a patient."