How to Start Insulin in Type 2 Diabetes
When to Initiate Insulin Immediately
Start insulin right away—without waiting—when patients present with severe hyperglycemia or metabolic decompensation. Specifically, initiate insulin immediately when HbA1c ≥9%, fasting glucose ≥11.1 mmol/L (≈200 mg/dL), random glucose consistently >300 mg/dL, or when HbA1c ≥10-12% with symptomatic hyperglycemia (polyuria, polydipsia, weight loss) or catabolic features 1, 2. In these scenarios, begin with a basal-bolus regimen using 0.3-0.5 units/kg/day total daily dose, split 50% basal and 50% prandial insulin 1, 2.
When to Add Insulin After Oral Agent Failure
Add basal insulin when HbA1c remains above target after 3 months of optimized oral medications (metformin plus additional agents) and lifestyle modifications 3, 1, 4. The threshold for definite insulin consideration is HbA1c ≥9%, with essential need at ≥10% 1.
Step 1: Choose Basal Insulin as First-Line
Begin with once-daily basal insulin (glargine, detemir, or degludec) at 10 units once daily OR 0.1-0.2 units/kg/day, administered at the same time each day—typically at bedtime 1, 2, 4. Long-acting analogs are preferred over NPH because they cause less nocturnal hypoglycemia when titrated to the same fasting target 4, 5.
Critical Foundation Therapy
Continue metformin at maximum tolerated dose (up to 2000-2550 mg daily) when starting insulin 1, 2, 4. Metformin reduces total insulin requirements by 20-30%, limits weight gain, and provides superior glycemic control compared to insulin alone 1, 2, 4. Discontinue sulfonylureas when initiating insulin to avoid additive hypoglycemia risk 1, 4.
Step 2: Titrate Basal Insulin to Target
Use a systematic every-3-day titration protocol based on fasting glucose patterns:
- Fasting glucose 140-179 mg/dL → increase basal insulin by 2 units every 3 days 1, 2, 4
- Fasting glucose ≥180 mg/dL → increase basal insulin by 4 units every 3 days 1, 2, 4
- Target fasting glucose: 80-130 mg/dL 1, 2, 4
- If hypoglycemia (<70 mg/dL) occurs → reduce dose by 10-20% immediately 1, 2, 4
Daily fasting glucose monitoring is essential during titration—patients should check each morning and adjust accordingly 1, 2, 4.
Step 3: Recognize When to Stop Basal Escalation
Stop increasing basal insulin when the dose reaches 0.5-1.0 units/kg/day without achieving glycemic targets 1, 2, 4. Continuing beyond this threshold causes "over-basalization"—a dangerous pattern where excessive basal insulin masks the need for mealtime coverage, leading to increased hypoglycemia without improved control 1, 2.
Clinical Signals of Over-Basalization
- Basal dose >0.5 units/kg/day 1, 2
- Bedtime-to-morning glucose differential ≥50 mg/dL 1, 2
- Recurrent hypoglycemia episodes 1, 2
- High day-to-day glucose variability 1, 2
Step 4: Add Prandial Insulin When Needed
Add rapid-acting insulin (lispro, aspart, or glulisine) when:
- Fasting glucose is at target (80-130 mg/dL) but HbA1c remains above goal after 3-6 months 1, 2, 4
- Basal insulin dose is 0.5-1.0 units/kg/day without achieving HbA1c targets 1, 2, 4
- Significant postprandial excursions (>180 mg/dL) persist despite basal optimization 1, 2, 4
Prandial Insulin Initiation
Start with 4 units of rapid-acting insulin before the largest meal OR 10% of the current basal dose 1, 2. Administer 0-15 minutes before meals (ideally immediately before eating) 1, 2, 5. Titrate each meal dose by 1-2 units (≈10-15%) every 3 days based on 2-hour postprandial glucose, targeting <180 mg/dL 1, 2.
Alternative: Consider GLP-1 Receptor Agonist Instead of Prandial Insulin
When basal insulin exceeds 0.5 units/kg/day, adding a GLP-1 receptor agonist (instead of prandial insulin) provides comparable postprandial control with less hypoglycemia and weight gain 1, 2. This combination (basal insulin + GLP-1 RA) is particularly effective for patients concerned about weight or hypoglycemia 1, 2.
Common Pitfalls to Avoid
Never delay insulin initiation in patients failing to meet glycemic goals with oral medications—prolonged hyperglycemia increases complication risk 1, 2, 4. Never discontinue metformin when starting insulin unless contraindicated—this leads to higher insulin requirements and greater weight gain 1, 2, 4. Never continue escalating basal insulin beyond 0.5-1.0 units/kg/day without addressing postprandial hyperglycemia—this causes over-basalization with increased hypoglycemia risk and suboptimal control 1, 2, 4.
Never use sliding-scale insulin as monotherapy—all major diabetes guidelines condemn this reactive approach as ineffective and dangerous 1, 2. Correction doses must supplement (not replace) scheduled basal and prandial insulin 1, 2.
Expected Outcomes
With properly implemented basal insulin therapy, approximately 68% of patients achieve mean glucose <140 mg/dL, compared with only 38% using sliding-scale insulin alone 1, 2. HbA1c reductions of 1.5-2.0% are achievable with basal insulin added to metformin 1. When basal-bolus therapy is needed for severe hyperglycemia, HbA1c reductions of 3-4% are observed over 3-6 months with intensive titration 1, 2.
Patient Education Essentials
Provide comprehensive education on:
- Insulin injection technique and site rotation to prevent lipohypertrophy 1, 2, 5
- Hypoglycemia recognition and treatment (symptoms, <70 mg/dL threshold, 15-g carbohydrate rule) 1, 2
- Self-monitoring of blood glucose—at least daily fasting checks during titration 1, 2, 4
- "Sick day" management rules—continue insulin even if not eating, check glucose every 4 hours, maintain hydration 1, 2
- Insulin storage and handling 1, 2