complication (medicine) complication (medicine) a complication in medicine, or medical complication, is an unfavorable result of a disease, health condition, or treatment. complications may adversely affect the prognosis, or outcome, of a disease. complications generally involve a worsening in severity of disease or the development of new signs, symptoms, or pathological changes which may become widespread throughout the body and affect other organ systems. thus, complications may lead to the development of new diseases resulting from a previously existing disease. complications may also arise as a result of various treatments.