In medicine, an Adverse effect is an abnormal, harmful, undesired and/or unintended side-effect, although not necessarily unexpected, which is obtained as a result of a therapy or other medical intervention, such as drug/chemotherapy, physical therapy, surgery, medical procedure, use of a medical device, etc. Iatrogenesis (literally, generated by a physician) is a common cause of adverse effects, as well as medical error. Using a drug or other medical intervention which is contraindicated may increase the risk of adverse effects. Adverse effects may cause medical complications of a disease or procedure and negatively affect its prognosis.

The harmful outcome is usually indicated by some result such as morbidity, mortality, alteration in body weight, levels of enzymes, loss of function or as a pathological change detected at the microscopic, macroscopic or physiological level. They may cause a reversible or irreversible change, including an increase or decrease in the susceptibility of the individual to other chemicals, foods, or procedures (e.g. drug interaction).

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