Internal medicine is the medical specialty dealing with the prevention, diagnosis, and treatment of adult diseases. Physicians specializing in internal medicine are called internists, or physicians (without a modifier) in Commonwealth nations. Internists are skilled in the management of patients who have undifferentiated or multi-system disease processes. Internists care for hospitalized and ambulatory patients and may play a major role in teaching and research.

Because internal medicine patients are often seriously ill or require complex investigations, internists do much of their work in hospitals. Internists often have subspecialty interests in diseases affecting particular organs or organ systems.

Internal medicine is also a specialty within clinical pharmacy and veterinary medicine.

The etymology of the term internal medicine in English is rooted in the German term Innere Medizin from the 19th century.^[1] Internal medicine delved into underlying pathological causes of symptoms and syndromes by use of laboratory investigations in addition to bedside clinical assessment of patients. In contrast, physicians in previous generations, such as the 17th-century physician Thomas Sydenham, who is known as the father of English medicine or "the English Hippocrates", had developed nosology (the study of diseases) via the clinical approach to diagnosis and management, by careful bedside study of the natural history of diseases and their treatment.^[2] Sydenham eschewed dissection of corpses and scrutiny of the internal workings of the body, for considering the internal mechanisms and causes of symptoms.^[3] It was thus subsequent to the 17th century that there was a rise in anatomical pathology and laboratory studies, with Giovanni Battista Morgagni, an Italian anatomist of the 18th century, being considered the father of anatomical pathology.^[4] Laboratory investigations became increasingly significant, with contribution of doctors including German physician and bacteriologist Robert Koch in the 19th century.^[5] The 19th century saw the rise of internal medicine that combined the clinical approach with use of investigations.^[6] Many early-20th century American physicians studied medicine in Germany and brought this medical field to the United States. Thus, the name "internal medicine" was adopted in imitation of the existing German term.^[7]

Historically, some of the oldest traces of internal medicine can be traced from Ancient India and Ancient China.^[8] Earliest texts about internal medicine are the Ayurvedic anthologies of Charaka.^[9]

Internal medicine specialists, also known as general internal medicine specialists or general medicine physicians in Commonwealth countries,^[10] are specialist physicians trained to manage particularly complex or multisystem disease conditions that single-organ-disease specialists may not be trained to deal with.^[11] They may be asked to tackle undifferentiated presentations that cannot be easily fitted within the expertise of a single-organ specialty,^[12] such as dyspnoea, fatigue, weight loss, chest pain, confusion or change in conscious state.^[13] They may manage serious acute illnesses that affect multiple organ systems at the same time in a single patient, and they may manage multiple chronic diseases or "comorbidities" that a single patient may have.^[14]

General internal medicine specialists do not provide necessarily less expertise than single-organ specialists, rather, they are trained for a specific role of caring for patients with multiple simultaneous problems or complex comorbidities.^[15]

Perhaps because it is complex to explain treatment of diseases that are not localised to a single-organ, there has been confusion about the meaning of internal medicine and the role of an "internist."^[16] Internists are qualified physicians with postgraduate training in internal medicine and should not be confused with "interns",^[17] who are doctors in their first year of residency training (officially the term intern is no longer in use).^[18][19] Although internists may act as primary care physicians, they are not "family physicians," "family practitioners," or "general practitioners," whose training is not solely concentrated on adults and may include surgery, obstetrics, and pediatrics. The American College of Physicians defines internists as "physicians who specialize in the prevention, detection and treatment of illnesses in adults".^[20]

Internal medicine physicians have practiced both in clinics and in hospitals, often in the same day. Pressures on time have led to many internal medicine physicians to choose one practice setting, who may choose to practice only in the hospital, as a "hospitalist", or only in an outpatient clinic, as a primary care physician.^[21]

The training and career pathways for internists vary considerably across the world.

Many programs require previous undergraduate education prior to medical school admission. This "pre-medical" education is typically four or five years in length. Graduate medical education programs vary in length by country. Medical education programs are tertiary-level courses, undertaken at a medical school attached to a university. In the United States, medical school consists of four years. Hence, gaining a basic medical education may typically take eight years, depending on jurisdiction and university.

Following completion of entry-level training, newly graduated medical practitioners are often required to undertake a period of supervised practice before the licensure, or registration, is granted, typically one or two years. This period may be referred to as "internship", "conditional registration", or "foundation programme". Then, doctors may finally follow specialty training in internal medicine if they wish, typically being selected to training programs through competition. In North America, this period of postgraduate training is referred to as residency training, followed by an optional fellowship if the internist decides to train in a subspecialty. In Commonwealth countries, during that training period in internal medicine, trainees are often called senior house officers, and advance to registrar grade when they undergo a compulsory subspecialty training whilst commonly continuing service provision in the main speciality. In the United States, residency training for internal medicine lasts three years.^[22][23]

In the United States, three organizations are responsible for certification of trained internists (i.e., doctors who have completed an accredited residency training program) in terms of their knowledge, skills, and attitudes that are essential for excellent patient care: the American Board of Internal Medicine, the American Osteopathic Board of Internal Medicine and the Board of Certification in Internal Medicine.

In the United States, two organizations are responsible for certification of subspecialists within the field: the American Board of Internal Medicine and the American Osteopathic Board of Internal Medicine. Physicians (not only internists) who successfully pass board exams get "board certified" status.

The following are the subspecialties recognized by the American Board of Internal Medicine.^[24]

The American College of Osteopathic Internists recognizes the following subspecialties:^[26]

In the United Kingdom, the three medical Royal Colleges (the Royal College of Physicians of London, the Royal College of Physicians of Edinburgh and the Royal College of Physicians and Surgeons of Glasgow) are responsible for setting curricula and training programmes through the Joint Royal Colleges Postgraduate Training Board (JRCPTB), although the process is monitored and accredited by the General Medical Council (which also maintains the specialist register).

Doctors who have completed medical school spend two years in foundation training completing a basic postgraduate curriculum. After two years of Core Medical Training (CT1/CT2) and attaining the Membership of the Royal College of Physicians, physicians commit to one of the medical specialties:^[27]

Many training programmes provide dual accreditation with general (internal) medicine and are involved in the general care to hospitalised patients. These are acute medicine, cardiology, Clinical Pharmacology and Therapeutics, endocrinology and diabetes mellitus, gastroenterology, infectious diseases, renal medicine, respiratory medicine and often, rheumatology. The role of general medicine, after a period of decline, was reemphasised by the Royal College of Physicians of London report from the Future Hospital Commission (2013).^[28]

Medicine is mainly focused on the art of diagnosis and treatment with medication, but many subspecialties administer procedural treatment: