Insulin is peptide hormone produced in the pancreas. The release of this hormone in the human body is most closely tied to blood glucose levels, although a number of other factors including pancreatic and gastrointestinal hormones, amino acids, fatty acids, and ketone bodies are also  involved. The main biological role of insulin is to promote the intracellular utilization and storage of amino acids, glucose, and fatty acids, while simultaneously inhibiting the breakdown of glycogen, protein, and fat. It is most notably identified with the control of blood sugar levels, and insulin medications are typically prescribed to people with diabetes, a metabolic disorder characterized by high  blood  sugar. While insulin targets many organs in the body, this hormone is both anabolic and anti-catabolic to muscle tissue. Insulin first became available as a medicine during the 1920’s. The first insulin preparations made were produced from crude pancreatic extracts taken from dogs. At one point when the supply of dead dogs ran out, the group actually started taking stray dogs off the streets in order to use. Soon after it was  discovered that cows and pigs worked as well that had been slaughtered and this made the research and development much more convenient and acceptable to the public. In 1974, chromatographic purification techniques allowed the manufacture of animal insulin with extreme purity. The pharmaceutical company Novo was the first to release a drug made with this technology.

There are three types of non-prescription insulin. Fast-acting, which is mostly used, known as Humulin-R. Then there is an intermediate form (Humulin-N or Humulin-L) which can last almost three times as long, which means up to a day. And lastly there is the Humulin-U, which stays active for longer. Particularly useful for diabetics who may forget their shots, as it stays active longer than a day. There is also a really fast-acting form called Humalog, but this is only available via prescription since it's the most easily abused and the Humulin-R suffices for most diabetics. Humulin-R is the compound most used by the way because it's the shortest acting form. This is considered to be good because it allows the user to quickly compensate for what needs to be done in terms of carbohydrate intake. Initially, doses of insulin will make you leaner as you store more carbs that would otherwise be stored as fat. But as people will tell you, it eventually has a tendency to make you fat. As indicated earlier, there is no negative feedback, but cells develop a resistance to insulin, in which case circulating excess carbs will be processed as adipose tissue. And if you know what's good for you, you will have circulating extra carbs.

 When administering supra-physiological doses of insulin, more glucose is stored as glycogen resulting in a lower blood sugar level. When your blood sugar level is too low, its called hypoglycemia and it can cause you to go into shock and die. If proper protocol for using insulin is not followed, you can go into a diabetic coma and possibly die. This has two definite implications. First of all it explains why you want the short-acting form. Blood sugar levels need to be monitored over the active time, so you obviously don't want it to stay active for 24 hours or longer. The second implication is that obviously sugar has to be taken with the insulin to prevent hypoglycemia and sugar needs to be kept on hand for the entire duration of activity, which is 6-8 hours. If dizziness or weakness occurs, more sugar has to be taken.

Insulin dosage will vary from person to person depending on a large number of factors such as bodyweight, insulin sensitivity, use of other drugs, activity level, and diet. The most common dosing protocol is to use 1IU of the drug for every 15-20lbs of lean bodyweight, with 10IU being the most common dosage. First time cautious users usually ignore bodyweight guidelines, and instead start at a low dosage with intention of gradually working up to a normal dosage.