What Is Insulin?

What is insulin?

Insulin is a hormone produced in the pancreas that regulates glucose levels in the blood. It grants the cells of the body the ability of glucose absorption from the bloodstream and the utility and storage of this glucose for immediate energy or  later use. Insulin is vital for  the maintenance of normal blood sugar levels, and, as such, it is primarily  used in the treatment of diabetes.

Insulin is a polypeptide hormone that is released by the beta cells of the pancreas.1 It acts by increasing glucose uptake by adipose tissue and muscles and hindering hepatic (liver) glucose release. Insulin reduces the concentrations of blood glucose to prevent hyperglycaemia and its associated  microvascular, macrovascular, and metabolic complications.1

There is natural profile for the secretion of insulin; basal production (a steady but low background secretion of insulin, consistently released from the pancreas,  that regulates the glucose), and bolus insulin (secreted at mealtime when glucose is absorbed into the blood following food and drink consumption).1

What does insulin do? 

Sitting behind your stomach, your pancreas functions by releasing insulin to regulate the level of glucose in your blood2 insulin is a hormone that controls the metabolism of carbohydrates, fats, and protein within  the body. It functions primarily by lowering the amount of plasma glucose through the facilitation of glucose uptake by cells for energy and storage.  Insulin also functions by aiding the conversion of excess glucose to the storage form, glycogen, in the liver and muscle cells.  The hormone plays an essential role in the maintenance of optimal blood glucose levels and energy homeostasis in the body, alongside promoting the synthesis of essential proteins. After having a meal, the carbohydrate from your food is broken down to glucose in the intestines, which then moves into the bloodstream and causes the blood glucose level to increase. These blood glucose levels determine the production and secretion of insulin in a biological  feedback loop. In illustration, this is similar to  the action of the thermostat or air conditioning  in your home; as the ambient temperature risesor falls, heat or cool  air is released appropriately. As such, beta cells in the pancreas are stimulated to release insulin in the presence of high blood glucose. This means that the pancreas releases more insulin as your blood glucose increases.2

Insulin aids the movement of glucose into cells for the generation of energy- after this occurs, plasma glucose levels then return to normal. 

In a similar feedback mechanism, another group of cells in the pancreas (the alpha cells)  are prompted to release a different hormone called glucagon in the presence of low blood glucose. Stored glucose in the liver, glycogen, is broken down into glucose by glucagon and is released back into the bloodstream.2 Insulin and glucagon constantly work in opposite directions in regulating your blood glucose levels.

When is insulin prescribed?

Insulin is mainly prescribed to people with diabetes; a chronic condition characterized by systemic health problems as a result of consistent high blood glucose levels. In Type 1 diabetes, little or no insulin is produced by the pancreas, making external insulin therapy necessary for the regulation of blood glucose levels. In Type 2 diabetes, there is resistance to insulin by the body; insulin production by the pancreas may be insufficient to control glucose uptake to an appropriate level. In this situation, insulin therapy may be prescribed in supplementation to the body’s natural insulin production. In medical conditions where there is insulin deficiency or resistance, insulin may also be utilized. Insulin is prescribed by a healthcare provider and is administered through injection or an automatic insulin pump.

Insulin is typically prescribed shortly after a Type 1 diabetes diagnosis to prevent health complications from developing, and a lifetime prescription is often needed to maintain safe blood glucose levels..3 An insulin prescription may be required in people with Type 2 diabetes if diet and exercise alone are insufficient at  controlling their blood sugar level Subsequently, if adequate  glucose control is achieved, Type 2 diabetic people could stop taking  insulin.3

Types of insulin treatments

Insulin preparations have three broad categories based on their time-action;  short, intermediate, and long-action insulin. Each individual needs to be assessed before insulin is prescribed as the duration of action of each type of insulin varies from one person to another.1

Short acting insulin

Short-acting insulin has a rapid onset of action and a short duration for substitution of body insulin release following a meal. These are available in rapid-acting insulin analogues (insulin apart, insulin glulisine and insulin lispro) and rarely used soluble insulin (human, bovine or porcine).1

Rapid-acting insulin starts to affect  blood glucose about 15 minutes after injection, working for up to a few hours per dose with  peak uptake occurring after an hour.  . Short-acting soluble insulin enters the bloodstream within 30 minutes of injection, peaking at around  2–3 hours, and staying effective for about 3 to 6 hours in total.4

Intermediate acting insulin

Isophane insulin and biphasic insulin are intermediate-acting insulin that have intermediate duration of action, and are made to mimic the action of endogenous basal insulin.1 They help the control of blood sugar up to 10-12 hours.4

Long acting insulin

Long-acting insulins (protamine zinc insulin, insulin zinc suspension, insulin detemir, insulin glargine, insulin degludec) act like endogenous l insulin but have the achievement of a steady insulin level after 2-4 days.1 This type of insulin enters the bloodstream 1-2 hours after injection and stays effective up to 24 hours.4

Can insulin be taken without a syringe? 

Though syringe is the common method of taking insulin, other options are available.

These options include:

  • Insulin pens: These are pre-filled pens that have a cartridge of insulin. They resemble writing pens and help you to avoid under or overdosing yourself by providing an exact correct dosage. You don’t need to keep them in the fridge and you can use them at your convenience4
  • Insulin pumps: These are small devices that you can wear on your skin, and they deliver insulin through a thin tube. They are programmed to deliver continuous insulin dose or a bolus (set) dose before your meal4
  • Jet injection: These are devices that deliver insulin without needles. Jet injections use a short burst of high-pressure to spray insulin through the skin and into the blood.  They hold many doses of insulin, though are still relatively new and hard to acquire4
  • Inhaled insulin: These are newer forms of insulin that are rapid-acting and are inhaled via the use of devices like asthma inhalers. This insulin is absorbed through the lungs after inhalation4

You will need to monitor your blood glucose levels for an appropriate insulin dosing unless you have an insulin pump that functions like a glucose monitor. It is vital to note that the use of these alternatives should be discussed with a health care provider for appropriate dosing and administration.4

Side effects and reactions to insulin

Insulin is usually safe when it is used according to the stipulated direction, but, just like other medications, it has side effects and reactions in some rare cases.5 Some side effects of insulin are: hypoglycaemia (low blood sugar), weight gain, skin thickening or lumps at injection sites, and injection site reactions, such as redness, itching and swelling. Most insulin reactions result from wrong dosing and inappropriate injection practices.5 Serious reactions to insulin are severe allergic reactions which include anaphylaxis, lipodystrophy (shrinkage or thickening of fat tissue under the skin), and insulin resistance, where the body becomes less responsive to insulin in the long run. Hypokalaemia is another condition where blood potassium levels become too low. There is also a small risk of local or systemic infections developing at the injection site, so cleaning the area of injection first is essential.5

It is recommended that you speak to your healthcare provider as soon as possible if you experience any unusual symptoms or side effects from taking insulin medication.

Treatment of diabetes 

The treatment of diabetes basically involves acombination of lifestyle interventions, medication, and regular blood sugar monitoring. Insulin therapy is required to control blood sugar levels in some cases. Lifestyle interventions include eating healthy, engaging in regular physical activity, maintaining a healthy weight, and avoiding smoking and excessive alcohol consumption. Regular monitoring of blood sugar levels is essential to monitor the effectiveness of treatment for adjustments where needed.


Insulin is a hormone produced in the beta cells of the pancreas that acts by regulating the metabolism of carbohydrates, fats, and proteins in the body in the process of energy generation. Its essential function is lowering the amount of blood glucose through the facilitation of glucose uptake into cells either for energy or storage. Insulin is typically prescribed for the treatment of diabetes, a long-term health condition with characteristic high blood sugar levels. Lifestyle changes, medication, and regular blood sugar monitoring are utilised in the treatment of diabetes, and insulin therapy is often  essential in the control of blood sugar levels. There are different methods of insulin administration such as syringes, pens, pumps and inhalers. Insulin is often safe with the right dosing and administration, but can cause some side effects and reactions in a small number of people  Insulin must always be prescribed by a health provider. 


  1. BNF is only available in the UK [Internet]. NICE. [cited 2023 Feb 14]. Available from: https://www.nice.org.uk/bnf-uk-only 
  2. Physiologic Effects of Insulin. http://www.vivo.colostate.edu/hbooks/pathphys/endocrine/pancreas/insulin_phys.html.
  3. Watts M. Being prescribed with insulin can be daunting. This section on insulin prescriptions aims to answer insulin prescription questions. [Internet]. Diabetes. 2019 [cited 2023 Feb 16]. Available from: https://www.diabetes.co.uk/insulin/insulin-prescription.html
  4. Insulin: types of insulin, needles, pumps, pens, and why insulin is so expensive [Internet]. 2022 [cited 2023 Feb 16]. Available from: https://www.endocrineweb.com/conditions/type-1-diabetes/what-insulin
  5. Vieira G. Insulin side effects: what you need to know [Internet]. Diabetes Strong. 2019 [cited 2023 Feb 16]. Available from: https://diabetesstrong.com/insulin-side-effects/
This content is purely informational and isn’t medical guidance. It shouldn’t replace professional medical counsel. Always consult your physician regarding treatment risks and benefits. See our editorial standards for more details.

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Sandra Fidelis

Bachelor’s (Honours) Degree, Nursing Science, Nnamdi Azikiwe University

Sandra Fidelis is a registered nurse, certified health writer and public health researcher.
She has a wide range of experience in the care of diabetic patients, cancer patients, acutely ill patients, elderly care, clients with long-term conditions, palliative care, and public health care across various health systems with a bachelor’s degree in Nursing Science and continuing education in Public Health.
She brings her medical background to bear in her health content writing with the capacity of creating a layman’s impression of health articles and health content search engine optimization.

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