Insulin — Medication Guide, Uses & Safety — Uses, Dosage & Side Effects | MyMedicPlus

Insulin is a peptide hormone naturally produced and secreted by the beta cells of the pancreatic islets of Langerhans in response to rising blood glucose levels after eating. As a medication, insulin is the cornerstone of treatment for type 1 diabetes mellitus — where the immune system has destroyed the insulin-producing beta cells, making exogenous insulin replacement essential for survival — and is used in type 2 diabetes when other antidiabetic drugs are insufficient to achieve adequate blood glucose control. Insulin therapy has been available since 1922, when it was first used to save the life of a 14-year-old boy with type 1 diabetes. Modern insulin preparations are produced by recombinant DNA technology (identical to human insulin) or are engineered as insulin analogues with modified amino acid sequences to alter their pharmacokinetic profiles. Multiple formulations are available, classified by their onset of action, peak effect, and duration: rapid-acting analogues (insulin lispro, aspart, glulisine), short-acting regular human insulin, intermediate-acting (NPH insulin), and long-acting basal insulin analogues (glargine U100, glargine U300, detemir, degludec). Combination (premixed) insulins are also used. Insulin is administered by subcutaneous injection, via insulin pens (reusable or disposable), by continuous subcutaneous insulin infusion via an insulin pump (particularly in type 1 diabetes), or intravenously in hospital settings for diabetic emergencies. Insulin management is complex and always requires comprehensive diabetes education and ongoing supervision by an endocrinologist, diabetologist, or specialist diabetes nurse.

Insulin is the essential treatment for type 1 diabetes mellitus — it must be taken daily for life as the pancreas produces no insulin. It is also used in type 2 diabetes when lifestyle modification combined with oral and injectable antidiabetic agents cannot achieve adequate blood glucose control (typically as HbA1c progressively rises despite maximized oral therapy), or when there is a specific indication for early insulin use (e.g. very high blood glucose at diagnosis, acute illness, or significant beta cell failure). Insulin is the treatment of choice for gestational diabetes that cannot be controlled by dietary modification alone — metformin and some other oral agents may also be used in pregnancy in some guidelines, but insulin is the most widely accepted. Insulin is used in all patients with diabetic ketoacidosis (DKA) and in those with hyperosmolar hyperglycaemic state (HHS) requiring emergency hospitalization. Peri-operative blood glucose management in surgical patients, critically ill patients in intensive care units requiring tight glycaemic control, and patients on high-dose corticosteroids causing steroid-induced diabetes are other key indications. Type 2 diabetes patients may require insulin temporarily during severe acute illness, major surgery, pregnancy, or as part of a fixed mixture regimen to achieve better HbA1c targets.

Insulin binds to the insulin receptor (IR) — a transmembrane tyrosine kinase receptor — on the surface of cells throughout the body, most importantly in skeletal muscle, adipose tissue, and the liver. Receptor binding activates a signaling cascade (IRS-1 phosphorylation → PI3K → Akt pathway) that has multiple metabolic effects. In skeletal muscle and adipose tissue: insulin stimulates the translocation of glucose transporter 4 (GLUT4) from intracellular vesicles to the cell surface, dramatically increasing glucose uptake from the bloodstream into cells where it is used for energy (glycolysis) or stored as glycogen. In the liver: insulin suppresses hepatic glucose production (gluconeogenesis and glycogenolysis), promotes glycogen synthesis (glycogenesis), and stimulates de novo lipogenesis (conversion of excess glucose to fatty acids for storage as triglycerides). Insulin also suppresses the breakdown of fat (lipolysis) in adipose tissue and protein catabolism in muscle, promoting net anabolism. Insulin analogues have been engineered with specific amino acid modifications that alter their subcutaneous absorption kinetics: rapid-acting analogues (lispro, aspart, glulisine) are absorbed within 15 minutes and mimic the physiological mealtime insulin spike; long-acting analogues (glargine, detemir, degludec) form subcutaneous depots that dissolve slowly, providing a steady basal level of insulin over 20–42 hours, mimicking the continuous low-level insulin secretion of the healthy pancreas.

Insulin dosing is uniquely individualized and requires expert management — no two patients require the same dose or regimen. Doses depend on body weight, food intake, activity level, blood glucose targets, and sensitivity to insulin (which varies by time of day and with illness). Typical regimens in type 1 diabetes use a 'basal-bolus' approach: a long-acting basal insulin (e.g. insulin glargine/Lantus U100: typically 0.2–0.4 units/kg once daily; or insulin degludec/Tresiba for more flexibility) combined with rapid-acting bolus insulin with each meal (e.g. insulin aspart/NovoRapid or lispro/Humalog: typically 0.1–0.2 units/kg per meal, adjusted by carbohydrate counting). For type 2 diabetes, insulin is often added as a once-daily long-acting basal insulin starting at 10 units/day or 0.1–0.2 units/kg/day, titrated up by 2 units every 3 days until fasting glucose target is achieved. Insulin is injected subcutaneously into the abdomen, outer thigh, outer upper arm, or buttocks — rotating injection sites within the same region prevents lipohypertrophy (fatty lumps) which impair absorption. Rapid-acting insulin is injected 0–15 minutes before or immediately after starting a meal. Never use cloudy rapid-acting insulin — this indicates protein denaturation (except NPH which is deliberately cloudy). All insulin dose changes must be made with diabetes team guidance — never self-adjust doses significantly without consultation.

The most common, potentially dangerous, and most important side effect of insulin is hypoglycemia (low blood glucose — typically defined as blood glucose below 4.0 mmol/L or 70 mg/dL). Hypoglycemia can range from mild (shakiness, sweating, palpitations, tingling, hunger, anxiety — self-treatable with fast-acting carbohydrates) to moderate (confusion, difficulty concentrating, behavioral changes) to severe (seizures, loss of consciousness, requiring assistance from another person). Severe hypoglycemia is a medical emergency — administer glucagon (IM or nasal route), call emergency services, and do not give anything by mouth to an unconscious person. All patients on insulin must be taught to recognize and treat hypoglycemia, must carry fast-acting carbohydrate at all times (glucose tablets, glucose gel, fruit juice), and must have a glucagon emergency kit available for carers or family. Injection site reactions (local redness, itching, swelling) are common initially and usually resolve. Lipohypertrophy — fatty lumps from repeated injections at the same site — impairs insulin absorption unpredictably and is prevented by proper site rotation. Weight gain (averaging 2–4 kg) commonly accompanies initiation of insulin therapy, particularly in type 2 diabetes. Low blood potassium (hypokalemia) can occur with high doses of insulin, particularly in DKA treatment. Insulin U-500 (5 times more concentrated than standard U-100) — used in severe insulin-resistant patients — requires extreme caution as dosing errors can be life-threatening.

Insulin has a critically narrow therapeutic window — both insufficient and excessive dosing carry life-threatening risks (diabetic complications vs. severe hypoglycemia). Never switch insulin type, brand, concentration (e.g. U-100 to U-300 glargine), or delivery device without full guidance and instruction from your diabetes care team — even switching between two brands of glargine may require dose adjustment. Storage: unopened insulin vials and pens must be stored refrigerated at 2–8°C and must never be frozen (freezing destroys insulin). Once opened or in-use, most insulin pens and vials can be kept at room temperature (below 25–30°C) for 28–30 days — check product-specific instructions. Protect insulin from direct sunlight and extreme heat. Inspect insulin before each use — discard if discolored, cloudy (for clear insulins), or containing particles. Key drug interactions: beta-blockers can mask the warning symptoms of hypoglycemia (sweating and tachycardia — but not hunger and confusion) and may also prolong hypoglycemia duration; inform your cardiologist if you are on insulin. Corticosteroids (prednisolone, dexamethasone) significantly raise blood glucose and markedly increase insulin requirements — blood glucose must be monitored more frequently during steroid courses. Thiazide diuretics and some antipsychotics also raise blood glucose. Alcohol causes delayed hypoglycemia (often many hours after drinking) by blocking hepatic gluconeogenesis — monitor blood glucose closely after alcohol and never skip meals. All insulin-treated patients should use continuous glucose monitoring (CGM) or perform regular blood glucose self-monitoring and maintain a sick-day management plan (how to adjust insulin during illness). Medical identification (bracelet or card stating insulin-treated diabetic) is strongly recommended.