23rd January, 1922: The first injection of Insulin (called isletin originally) on Leonard Thompson, a 14-year-old diabetic who lay dying at the Toronto General Hospital on 11th January 1922 caused a severe allergic reaction. For the next 12 days the biochemist James Collip worked to improve and purify the ox-pancreas extract and on the second injection bought the boy out of his coma and was a complete success.

Insulin is a hormone that has extensive effects on metabolism and other body functions, such as vascular compliance. Insulin causes cells in the liver, muscle, and fat tissue to take up glucose from the blood, storing it as glycogen in the liver and muscle, and stopping use of fat as an energy source. When insulin is absent (or low), glucose is not taken up by body cells, and the body begins to use fat as an energy source, for example, by transfer of lipids from adipose tissue to the liver for mobilization as an energy source. As its level is a central metabolic control mechanism, its status is also used as a control signal to other body systems (such as amino acid uptake by body cells). It has several other anabolic effects throughout the body. When control of insulin levels fails, diabetes mellitus will result.

Insulin is used medically to treat some forms of diabetes mellitus. Patients with Type 1 diabetes mellitus depend on external insulin (most commonly injected subcutaneously) for their survival because the hormone is no longer produced internally. Patients with Type 2 diabetes mellitus are insulin resistant, and because of such resistance, may suffer from a relative insulin deficiency. Some patients with Type 2 diabetes may eventually require insulin when other medications fail to control blood glucose levels adequately.

Insulin is a peptide hormone composed of 51 amino acids and has a molecular weight of 5808 Da. It is produced in the islets of Langerhans in the pancreas. The name comes from the Latin insula for "island". Insulin's structure varies slightly between species of animal. Insulin from animal sources differs somewhat in 'strength' (in carbohydrate metabolism control effects) in humans because of those variations. Porcine (pig) insulin is especially close to the human version.

In 1869 Paul Langerhans, a medical student in Berlin, was studying the structure of the pancreas under a microscope when he identified some previously un-noticed tissue clumps scattered throughout the bulk of the pancreas. The function of the "little heaps of cells," later known as the Islets of Langerhans, was unknown, but Edouard Laguesse later suggested that they might produce secretions that play a regulatory role in digestion. Paul Langerhans' son, Archibald, also helped to understand this regulatory role. The term insulin origins from Insel, the German word for islet/island.

In 1889, the Polish-German physician Oscar Minkowski in collaboration with Joseph von Mering removed the pancreas from a healthy dog to test its assumed role in digestion. Several days after the dog's pancreas was removed, Minkowski's animal keeper noticed a swarm of flies feeding on the dog's urine. On testing the urine they found that there was sugar in the dog's urine, establishing for the first time a relationship between the pancreas and diabetes. In 1901, another major step was taken by Eugene Opie, when he clearly established the link between the Islets of Langerhans and diabetes: Diabetes mellitus … is caused by destruction of the islets of Langerhans and occurs only when these bodies are in part or wholly destroyed. Before his work, the link between the pancreas and diabetes was clear, but not the specific role of the islets.

Over the next two decades, several attempts were made to isolate whatever it was the islets produced as a potential treatment. In 1906 George Ludwig Zuelzer was partially successful treating dogs with pancreatic extract but was unable to continue his work. Between 1911 and 1912, E.L. Scott at the University of Chicago used aqueous pancreatic extracts and noted a slight diminution of glycosuria but was unable to convince his director of his work's value; it was shut down. Israel Kleiner demonstrated similar effects at Rockefeller University in 1919, but his work was interrupted by World War I and he did not return to it. Nicolae Paulescu, a professor of physiology at the University of Medicine and Pharmacy in Bucharest, was the first one to isolate insulin, which he called at that time pancrein, and published in 1921 the work that he had carried out in Bucharest. Use of his techniques was patented in Romania, though no clinical use resulted.
 
In October 1920 Canadian Frederick Banting was reading one of Minkowski's papers and concluded that it is the very digestive secretions that Minkowski had originally studied that were breaking down the islet secretion(s), thereby making it impossible to extract successfully. He jotted a note to himself Ligate pancreatic ducts of the dog. Keep dogs alive till acini degenerate leaving islets. Try to isolate internal secretion of these and relieve glycosurea. The idea was that the pancreas's internal secretion, which supposedly regulates sugar in the bloodstream, might hold the key to the treatment of diabetes. A surgeon by training, Banting knew that certain arteries could be tied off that would lead to atrophy of most of the pancreas while leaving the islets of Langerhans intact. He theorized that a relatively pure extract could be made from the islets once most of the rest of pancreas was gone.

In the spring of 1921 Banting traveled to Toronto to explain his idea to J.J.R. Macleod who was Professor of Physiology at the University of Toronto, and asked Macleod if he could use his lab space to test the idea. Macleod was initially skeptical, but eventually agreed to let Banting use his lab space while he was on vacation for the summer. He also supplied Banting with ten dogs to experiment on, and two medical students, Charles Best and Clark Noble, to use as lab assistants, before leaving for Scotland. Since Banting only required one lab assistant, Best and Noble flipped a coin to see which would assist Banting for the first half of the summer. Best won the coin toss, and took the first shift as Banting's assistant. Loss of the coin toss may have proved unfortunate for Noble, given that Banting decided to keep Best for the entire summer, and eventually shared half his Nobel Prize money and a large part of the credit for the discovery of insulin with the winner of the toss. Had Noble won the toss, his career might have taken a different path.[18] Banting's method was to tie a ligature (string) around the pancreatic duct, and, when examined several weeks later, the pancreatic digestive cells had died and been absorbed by the immune system, leaving thousands of islets. They then isolated an extract from these islets, producing what they called isletin (what we now know as insulin), and tested this extract on the dogs. Banting and Best were then able to keep a pancreatectomized dog named Alpha alive for the rest of the summer by injecting her with the crude extract they had prepared. Removal of the pancreas in test animals essentially mimics diabetes, leading to elevated blood glucose levels. Alpha was able to remain alive because the extracts, containing isletin, were able to lower her blood glucose levels.

Banting and Best presented their results to Macleod on his return to Toronto in the fall of 1921, but Macleod pointed out flaws with the experimental design, and suggested the experiments be repeated with more dogs and better equipment. He then supplied Banting and Best with a better laboratory, and began paying Banting a salary from his research grants. Several weeks later, it was clear the second round of experiments was also a success; and Macleod helped publish their results privately in Toronto that November. However, they needed six weeks to extract the isletin, which forced considerable delays. Banting suggested that they try to use fetal calf pancreas, which had not yet developed digestive glands; he was relieved to find that this method worked well. With the supply problem solved, the next major effort was to purify the extract. In December 1921, Macleod invited the biochemist James Collip to help with this task, and, within a month, the team felt ready for a clinical test.

On January 11, 1922, Leonard Thompson, a 14-year-old diabetic who lay dying at the Toronto General Hospital, was given the first injection of insulin. However, the extract was so impure that Thompson suffered a severe allergic reaction, and further injections were canceled. Over the next 12 days, Collip worked day and night to improve the ox-pancreas extract, and a second dose was injected on the 23rd. This was completely successful, not only in having no obvious side-effects, but in completely eliminating the glycosuria sign of diabetes.

Children dying from diabetic keto-acidosis were kept in large wards, often with 50 or more patients in a ward, mostly comatose. Grieving family members were often in attendance, waiting the (until then, inevitable) death. In one of medicine's more dramatic moments Banting, Best, and Collip went from bed to bed, injecting an entire ward with the new purified extract. Before they had reached the last dying child, the first few were awakening from their coma, to the joyous exclamations of their families. However, Banting and Best never worked well with Collip, regarding him as something of an interloper, and Collip left the project soon after.

Over the spring of 1922, Best managed to improve his techniques to the point where large quantities of insulin could be extracted on demand, but the preparation remained impure. The drug firm Eli Lilly and Company had offered assistance not long after the first publications in 1921, and they took Lilly up on the offer in April. In November, Lilly made a major breakthrough and were able to produce large quantities of highly refined, 'pure' insulin. Insulin was offered for sale shortly thereafter.

Purified animal-sourced insulin was the only type of insulin available to diabetics until genetic breakthroughs occurred later with medical research. The amino-acid structure of insulin was characterized in the 1950s and the first synthetic insulin was produced simultaneously in the labs of Panayotis Katsoyannis at the University of Pittsburgh and Helmut Zahn at RWTH Aachen University in the early 1960s. The first genetically-engineered, synthetic "human" insulin was produced in a laboratory in 1977 by Herbert Boyer using E. coli. Partnering with Genentech founded by Boyer, Eli Lilly went on in 1982 to sell the first commercially available biosynthetic human insulin under the brand name Humulin. The vast majority of insulin currently used worldwide is now biosynthetic recombinant "human" insulin or its analogs.