When should insulin therapy be initiated in a patient with type 2 diabetes mellitus (T2DM) who has not achieved adequate glycemic control with lifestyle modifications and oral medications?

When to Start Insulin in Type 2 Diabetes

Initiate insulin therapy immediately in patients with newly diagnosed T2DM when HbA1c ≥9% or fasting glucose ≥11.1 mmol/L (≥200 mg/dL), especially if symptomatic hyperglycemia or catabolic features are present. 1, 2

Immediate Insulin Initiation Criteria

For severe hyperglycemia at diagnosis:

• Start insulin when random blood glucose consistently exceeds 300 mg/dL (16.7 mmol/L), combined with metformin and lifestyle modifications 1
• Begin insulin when HbA1c ≥10-12% with symptomatic hyperglycemia or catabolic features (weight loss, polyuria, polydipsia) 3, 1
• This approach allows rapid glucose normalization and gives beta cells a chance to "rest and recover" 1

Short-term intensive insulin therapy (2 weeks to 3 months):

• Implement in newly diagnosed patients with HbA1c >9.0% or fasting glucose ≥11.1 mmol/L with symptomatic hyperglycemia 3, 2
• Use basal insulin (glargine or detemir) at 0.2-0.3 units/kg/day, given once daily at bedtime 1
• Taper insulin after glucose control is achieved, typically within 2 weeks to 3 months 1, 2

Delayed Insulin Initiation (After Oral Agent Failure)

When oral medications fail to achieve glycemic targets:

• Add insulin when HbA1c remains above target after 3 months of optimal oral medications and lifestyle modifications 3, 1, 4
• Consider insulin at HbA1c ≥9%, with definite consideration at ≥10-12% 1
• Insulin is essential when HbA1c ≥10% (≥86 mmol/mol) despite optimized diet, physical activity, and other antihyperglycemic agents 4

The 3-month rule:

• Patients not achieving glycemic goals with lifestyle intervention and oral agents should initiate insulin as soon as possible, ideally within 3 months of recognized treatment failure 3, 2

Practical Implementation

Initial insulin regimen:

• Start with basal insulin (NPH, glargine, detemir, or degludec) as the preferred initial approach 3, 5
• For insulin-naïve patients inadequately controlled on oral agents, begin at 0.1-0.2 units/kg once daily in the evening or 10 units once or twice daily 6, 5
• Alternative: Premixed insulin 1-3 times daily for patients requiring more comprehensive coverage 3, 4

Continue metformin:

• Metformin should be continued unless contraindicated, as it reduces all-cause mortality, cardiovascular events, weight gain, insulin dose requirements, and hypoglycemia risk 1, 7
• Start metformin at 500 mg once daily with dinner, increasing by 500 mg every 1-2 weeks up to 2000 mg daily in divided doses 1

Titration Strategy

Dose adjustment protocol:

• Titrate insulin every 3-4 days based on fasting glucose targets (80-130 mg/dL) 1, 5
• Increase by 2-4 units every 3 days until fasting glucose of 80-130 mg/dL is reached 1
• Prophylactic titration algorithm: fasting glucose <4.4 mmol/L: -2U; 4.4-7.0: +0U; 7.1-10.0: +2U; >10.0: +4U 8

Monitoring requirements:

• Self-monitor blood glucose with fasting and 2-hour post-meal readings daily while on insulin 1
• Check HbA1c at 3 months to assess response 1
• Weekly follow-up initially to titrate insulin and assess for hypoglycemia 1

Target Goals

Glycemic targets:

• HbA1c <7% for most patients 1, 5
• Fasting glucose 80-130 mg/dL 1, 5
• 2-hour post-meal glucose <180 mg/dL 1, 5

Common Pitfalls to Avoid

Do not delay insulin unnecessarily:

• The progressive nature of T2DM means many patients will eventually require insulin 3
• Avoid using insulin as a threat or describing it as failure or punishment 3
• Providers should regularly and objectively explain the progressive nature of T2DM to patients 3

Do not abruptly discontinue oral medications:

• Oral medications should not be stopped abruptly when starting insulin due to risk of rebound hyperglycemia 4
• Continue metformin specifically for its mortality and cardiovascular benefits 1, 7

Watch for hypoglycemia and weight gain:

• Insulin use is associated with hypoglycemia risk and weight gain of approximately 3 kg over 6 months 8, 5
• Educate patients on identifying and responding to hypoglycemia symptoms 8
• Keep glucose tablets or sugar readily available 1