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.

The development of complications depends on a number of factors, including the degree of vulnerability, susceptibility, age, health status, and immune system condition. Knowledge of the most common and severe complications of a disease, procedure, or treatment allow for prevention and preparation for treatment if they should occur.

Complications are not to be confused with sequelae, which are residual effects that occur after the acute (initial, most severe) phase of an illness or injury. Sequelae can appear early in the development of disease or weeks to months later and are a result of the initial injury or illness. For example, a scar resulting from a burn or dysphagia resulting from a stroke would be considered sequelae. In addition, complications should not be confused with comorbidities, which are diseases that occur concurrently but have no causative association. Complications are similar to adverse effects, but the latter term is typically used in pharmacological contexts, or when the negative consequence is expected or common.