Complication, in medicine, is an unfavorable evolution of a disease, a health condition or a therapy. The disease can become worse in its severity or show a higher number of signs, symptoms or new pathological changes, become widespread throughout the body or affect other organ systems. A new disease may also appear as a complication to a previous existing disease. A medical treatment, such as drugs or surgery may produce adverse effects and/or produce new health problem(s) by itself. Therefore, a complication may be iatrogenic, i.e., literally brought forth by the physician.
Medical knowledge about a disease, procedure or treatment usually entails a list of the most common complications, so that they can be foreseen, prevented or recognized more easily and speedily.
Depending on the degree of vulnerability, susceptibility, age, health status, immune system condition, etc. complications may arise more easily. Complications affect adversely the prognosis of a disease. Non-invasive and minimally invasive medical procedures usually favor fewer complications in comparison to invasive ones.
Examples of complications 
- Generalized septicemia (infection of the blood) may occur as a complication of an infected wound or abscess
- Allergic shock can be a reaction to several kinds of anesthetics, as a complication in a surgery
- Fractured ribs and sternum may be a complication of cardiopulmonary resuscitation attempts in people suffering severe osteoporosis
- Puerperal fever may be a common complication of childbirth and used to kill a large proportion of mothers before the advent of antisepsis and antibiotics
- Diabetes mellitus may present a series of complications in an advanced or more severe stage, such as gangrene, diabetic foot, blindness, infections, etc.
- Thrombosis in the heart or brain, causing stroke or acute myocardial infarction can be complications of blood coagulation disorders, phlebitis (inflammation of the veins), endocarditis and artificial heart valves
- Eczema vaccinatum is a rare and severe complication of smallpox vaccination in people with eczema
- Hepatotoxic dementia is a possible complication of hepatitis and liver cirrhosis
- Mental retardation is a common complication of untreated hydrocephalus
- A paradoxical reaction to a drug; that is, a reaction that is the opposite to the intended purpose of the drug. An example is benzodiazepines, a class of psychoactive drugs considered minor tranquilizers with varying hypnotic, sedative, anxiolytic, anticonvulsant, and muscle relaxant effects; paradoxically they may also create hyperactivity, anxiety, convulsions etc. in susceptible individuals. 
- Erectile dysfunction and urinary incontinence which may follow prostatectomy.[medical citation needed]
Related topic 
- Adverse effect (medicine)
- Classification of Pharmaco-Therapeutic Referrals
- Placebo (origins of technical term)
- Late effect