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An epiphenomenon, medically speaking, is a secondary or additional symptom that a patient develops during the course of a disease and which may or may not be related to the disease in question. It can also refer to a secondary symptom or other occurrence that might mask the true cause-and-effect relationship under study. For example, a medication might seem to raise risk for a certain disease but, in fact, it may be the condition that was the reason for taking the medication in the first place that actually raises risk for the disease.
In the common definition of epiphenomena outside the medical setting, it is usually implied that an epiphenomenon is caused by the primary phenomenon — the phenomenon currently under study. It is also usually implied that the epiphenomenon cannot influence the primary phenomenon. Neither of these is necessarily true in the medical definition. Epiphenomena in medicine can be caused accidentally or directly by the primary phenomenon, though they may or may not appear to be related to it. They can also be completely unconnected to the condition in question and, in fact, can be symptoms of a concurrent disorder — that is, a disorder that occurs at the same time as the one currently under study.
An epiphenomenon that appears in either a clinical or research setting can be confusing, because its existence makes causal relationships unclear. Medical staff and patients may be uncertain if the epiphenomenon is a side effect of the primary phenomenon or a totally new condition. They might not be able to tell if the secondary symptom was caused by the disease, by the treatment that is being used for the disease, or by something else totally unrelated to the disease and its treatment. This is particularly complicated when it makes logical sense to either the medical staff or the patient that the secondary symptom would be related to the primary condition.
Dreaming is an example of an epiphenomenon that has been talked about and studied in medical research. It is unclear whether dreaming is simply a byproduct of sleep or serves any purpose in and of itself. In a bid to find out more about the relationship between sleep and dreaming, some studies look at dreaming in normal subjects versus dreaming in narcoleptic subjects or other people who have sleep- or dream-related problems. Researchers must be creative in finding ways to determine the true causality behind dreaming, just as they must be in determining the causality behind any epiphenomenon.