Insulin therapy is an essential component of diabetes management and remains the only effective treatment for individuals with type 1 diabetes mellitus. It is also indicated in many patients with type 2 diabetes mellitus when glycaemic targets cannot be achieved with lifestyle modification and oral hypoglycaemic agents alone or when insulin deficiency becomes clinically significant.

The aims of insulin therapy are to replace deficient endogenous insulin, achieve optimal glycaemic control, relieve symptoms of hyperglycaemia, prevent acute metabolic emergencies, reduce the risk of chronic complications, and improve quality of life.

Successful insulin therapy requires individualized dose adjustment, patient education, regular blood glucose monitoring, and careful attention to diet, physical activity, and injection technique.

Physiology of insulin

Insulin is a peptide hormone produced by the β-cells of the pancreatic islets of Langerhans. It promotes glucose uptake into skeletal muscle and adipose tissue, suppresses hepatic glucose production, stimulates glycogen synthesis, promotes protein synthesis, and inhibits lipolysis and ketogenesis.

Normal insulin secretion consists of:

• Basal insulin secretion, which maintains glucose homeostasis during fasting.

• Prandial (bolus) insulin secretion, which controls postprandial blood glucose following meals.

Insulin therapy aims to mimic these physiological patterns.

Indications for insulin therapy

Type 1 diabetes mellitus

All patients with type 1 diabetes mellitus require lifelong insulin therapy because of absolute insulin deficiency.

Type 2 diabetes mellitus

Insulin should be initiated in patients with:

• Fasting blood glucose greater than 15 mmol/L at presentation

• Hyperglycaemic emergencies

• Severe symptomatic hyperglycaemia

• Major surgery or the perioperative period

• Serious medical illnesses requiring tight glycaemic control

• Renal failure

• Liver failure

• Heart failure

• Poor glycaemic control despite lifestyle modification and oral hypoglycaemic agents

• Latent autoimmune diabetes in adults (LADA)

• Contraindications to oral hypoglycaemic agents

Clinical note

The maximum glucose-lowering effect of oral hypoglycaemic agents is usually evident within six months. Patients who remain uncontrolled despite appropriate dose titration should have their treatment intensified rather than continuing ineffective therapy.

Types of insulin

Insulin preparations differ according to their onset, peak, and duration of action.

Short-acting insulin

Short-acting insulin controls postprandial blood glucose excursions and is commonly used before meals or intravenously during diabetic emergencies and perioperative care.

Examples include:

• Soluble human insulin

Intermediate-acting insulin

Intermediate-acting insulin provides basal insulin coverage throughout the day and overnight.

Example:

• Neutral protamine Hagedorn (NPH) insulin

Premixed insulin

Premixed insulin combines intermediate-acting and short-acting insulin in a fixed ratio, providing both basal and mealtime insulin coverage.

Common preparation:

• Human insulin 70/30

Initiation of insulin therapy

Insulin should be initiated by a clinician experienced in diabetes management who is able to educate the patient regarding:

• Injection technique

• Dose adjustment

• Storage of insulin

• Blood glucose monitoring

• Recognition of hypoglycaemia

• Sick-day management

Once stabilized, patients may continue follow-up and prescription refills at lower-level health facilities.

Insulin as substitution therapy

Substitution therapy replaces endogenous insulin when oral hypoglycaemic agents are no longer adequate.

Oral medications

• Discontinue oral hypoglycaemic agents.

• Metformin may be continued in obese patients if appropriate.

Starting dose

Premixed insulin:

0.2 IU/kg/day

Divide the total daily dose into:

• Two-thirds before breakfast

• One-third before the evening meal

Administration

Human insulin:

• Inject approximately 30 minutes before meals

Insulin analogues:

• Inject 0–15 minutes before meals

Dose adjustments should be based on fasting and postprandial blood glucose measurements.

Insulin as supplemental therapy

Supplemental insulin is added to existing oral therapy when oral agents alone fail to achieve adequate glycaemic control.

Regimen

Continue:

• Metformin (up to 2 g/day)

• Sulphonylurea (usually half of the maximum dose)

Add:

NPH insulin

Dose:

0.1–0.2 IU/kg once daily at bedtime (before 22:00 hours)

Regular blood glucose monitoring should guide subsequent dose adjustments.

Insulin therapy in type 1 diabetes mellitus

Because endogenous insulin production is absent, lifelong insulin replacement is mandatory.

Recommended total daily insulin dose:

Before puberty

0.5 IU/kg/day

During puberty

1–2 IU/kg/day

After puberty

Usually less than 2 IU/kg/day

Individual requirements vary according to weight, pubertal stage, illness, diet, and physical activity.

Basal-bolus insulin regimen

The total daily insulin dose is divided into basal and prandial components.

This regimen closely mimics normal physiological insulin secretion and provides the best glycaemic control for most patients with type 1 diabetes.

Premixed insulin is generally not recommended in children because it offers less flexibility for meal timing and insulin adjustment.

Administration of insulin

Insulin is administered by subcutaneous injection into:

• Abdomen

• Anterior thigh

• Upper arm

• Buttocks

Injection sites should be rotated regularly to reduce the risk of lipohypertrophy and ensure consistent insulin absorption.

Patients should receive practical instruction on:

• Injection technique

• Needle disposal

• Storage of insulin

• Rotation of injection sites

Monitoring during insulin therapy

Regular monitoring is essential to achieve glycaemic control while minimizing hypoglycaemia.

Monitoring includes:

• Self-monitoring of blood glucose

• HbA1c every three months until stable

• Weight

• Blood pressure

• Renal function

• Injection site inspection

• Review of hypoglycaemic episodes

• Review of adherence and injection technique

Dose adjustment

Insulin doses should be adjusted according to:

• Fasting blood glucose

• Postprandial blood glucose

• HbA1c

• Physical activity

• Dietary intake

• Intercurrent illness

• Pregnancy

• Weight changes

Dose adjustments should generally be gradual to reduce the risk of hypoglycaemia.

Adverse effects

Potential adverse effects include:

• Hypoglycaemia

• Weight gain

• Lipohypertrophy

• Lipoatrophy

• Injection site pain

• Local allergic reactions

• Peripheral oedema during initiation

Hypoglycaemia

Hypoglycaemia is the most common complication of insulin therapy.

Symptoms

• Sweating

• Tremor

• Hunger

• Palpitations

• Anxiety

• Confusion

• Blurred vision

• Dizziness

• Behavioural changes

• Seizures

• Loss of consciousness

Common causes

• Excess insulin

• Delayed or missed meals

• Excessive physical activity

• Alcohol consumption

• Incorrect insulin administration

Management

Mild hypoglycaemia:

• Administer 15–20 g of rapidly absorbed carbohydrate.

• Recheck blood glucose after 15 minutes.

• Repeat if necessary.

Severe hypoglycaemia:

• Intravenous dextrose or intramuscular glucagon (where available).

• Identify and correct the precipitating cause.

Patient education

Patients should receive education regarding:

• Nature and purpose of insulin therapy

• Injection technique

• Blood glucose monitoring

• Recognition of hypo- and hyperglycaemia

• Sick-day rules

• Storage of insulin

• Needle disposal

• Diet and meal planning

• Physical activity

• Foot care

• Importance of adherence and regular clinic attendance

Clinical pearls

• Insulin is mandatory for all patients with type 1 diabetes mellitus.

• In type 2 diabetes mellitus, insulin should be initiated promptly when oral therapy fails or when severe hyperglycaemia is present.

• Basal insulin is commonly introduced before progressing to more intensive insulin regimens.

• Premixed insulin provides both basal and prandial insulin coverage but offers less flexibility than basal-bolus therapy.

• Regular self-monitoring of blood glucose and periodic HbA1c assessment are essential for safe insulin dose adjustment.

• Proper injection technique, site rotation, and patient education reduce complications and improve treatment outcomes.