Overdiagnosis and overtreatment are intertwined. There is debate about how to best describe the problem, but narrowly defined, overdiagnosis occurs when increasingly sensitive tests - or changing definitions - identify abnormalities that are minor, non-progressive, or likely to resolve on their own, and that, if left untreated, will not cause symptoms or shorten an individual's life. Having a diagnosis makes well-persons think they're ill (overdiagnosis). If they are then treated for this diagnosis, they are overtreated.
An example involving changing definitions pertains to diabetes. When the official definition of diabetes changed from having a fasting blood sugar (FBS) of "X" or greater to an FBS of "X-y" or greater, 1.6 million new diabetics were instantly diagnosed, some of whom are not likely to develop symptoms and complications and are not likely to benefit from treatment. This is overdiagnosis.
If one goes on to put such a patient (let's say, on the lower end, "X-y+1" on a blood sugar (BS) lowering agent, and they pass out because their BS drops too low, then they are overtreated. If they happened to be driving when they pass out, and get into a fatal accident, they literally die from overtreatment, if it can't be shown that they would have suffered from a marginally elevated FBS. (If it can be shown to be dangerous to have an FBS of X-y+1, then this is a risk of treatment, not overtreatment.)
For an example of increased sensitivity of tests, say a new test is available, a blood test called a "D-dimer", which will reveal with 100% accuracy the presence of a dangerous pulmonary embolism (it's very sensitive). However, it is also known to be positive when there is no pulmonary embolism as well, but it's not known how often (so it's not specific). But it's an exciting new test!
A patient goes into the Emergency room for one of the following: atypical chest pain, dizziness, shortness of breath, leg pain consistent with a blood clot in the calf, or loss of consciousness. In addition to an EKG, chest x-ray (CXR) and a few other tests, a D-dimer is ordered. The EKG, CXR and other tests are negative, but the D-Dimer is positive. The tentative diagnosis is pulmonary embolism (PE).
Because the D-dimer indicates that a dangerous condition may be present, a spiral CAT Scan with contrast is ordered on all those people. Some will have a life-threatening PE (appropriately diagnosed*). A large number will have a negative CAT Scan. (diagnosis excluded) But a significant number will have a very small PE. Because this is relatively new and surprising, and it's not known whether they should be treated (maybe nothing will happen if they don't treat? Maybe this happens in normal people and we just never knew it?), the doctors, who are accustomed to treating all PE's will put the patients on blood thinners. This is overdiagnosis of PE, and treating it is overtreatment, because some treated people will have "bleeds" - a stroke, GI bleeding, other. It is now known that these tiny PE's are often seen in patients who suffer no harm at all from them, therefore in order to stem overdiagnosis (and subsequent overtreatment), the D-dimer test is ordered less frequently.
...[We] are in the midst of an epidemic of diagnosis. Conventional wisdom tells us that finding problems early saves lives because we have the opportunity to fix the problems early. [Conventional wisdom tells us that} There is no risk in finding things early. The truth is that early diagnosis is a double-edged sword; while it has the potential to help some, it has a potential to harm us. Such overdiagnosis leads to overtreatment when these “pseudo-diseases” are conventionally managed and treated as if they were real abnormalities; because these findings have a benign prognosis, treatment can only do harm.
Overdiagnosed: Making People Sick in the Pursuit of Health
Using Evidence to Combat Overdiagnosis and Overtreatment: Evaluating Treatments, Tests, and Disease Definitions in the Time of Too Much