Internal medicine is the branch and specialty of medicine concerning the diagnosis and nonsurgical treatment of diseases in adults, especially of internal organs. Doctors of internal medicine, also called "internists", are required to have included in their medical schooling and postgraduate training at least three years dedicated to learning how to prevent, diagnose, and treat diseases that affect adults. Internists are sometimes referred to as the "doctor's doctor," because they are often called upon to act as consultants to other physicians to help solve puzzling diagnostic problems.

While the name "internal medicine" may lead one to believe that internists only treat "internal" problems, though this is not the case. Doctors of internal medicine treat the whole person, not just internal organs.

In the United Kingdom, the specialty is known as Acute Medicine

According to the American Medical Association, specialists of internal medicine earn between $175,000 to $350,000 per year.

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