For a patient taking insulin glargine (Lantus) 45 U with an insulin‑to‑carbohydrate ratio of 5 g per unit and an insulin sensitivity factor of 20 mg/dL per unit, how should the basal insulin dose, ICR, and ISF be adjusted when dapagliflozin (Farxiga) is added?

Insulin Dose Adjustment When Adding Farxiga to Lantus

Reduce your Lantus dose by 10-20% (approximately 4-9 units, bringing it to 36-41 units) when starting Farxiga, and closely monitor for hypoglycemia over the first few weeks, adjusting further as needed based on glucose patterns.

Basal Insulin Reduction

When adding dapagliflozin (Farxiga) to an existing insulin regimen, the risk of hypoglycemia increases due to the additive glucose-lowering effect. The 2025 ADA Standards recommend that if A1C is <8%, consider lowering the basal dose by 4 units per day or 10% of basal dose when adding new glucose-lowering agents 1.

For your current dose of 45 units:

• 10% reduction = 4.5 units (round to 4-5 units) → New dose: 40-41 units
• 20% reduction = 9 units → New dose: 36 units

Start with a 10% reduction (40 units) if your A1C is near goal or you've had any hypoglycemia. Use a 20% reduction (36 units) if your A1C is significantly above goal and you've had no hypoglycemia issues.

Research supports this approach: Studies combining SGLT2 inhibitors with basal insulin showed insulin dose reductions of 6-8 units were needed to maintain similar glycemic control while avoiding hypoglycemia 2, 3. One study specifically showed that when dapagliflozin was added to insulin regimens, daily insulin doses decreased by approximately 6-8 units compared to placebo groups 3.

Insulin Sensitivity Factor (ISF) Adjustment

Increase your ISF from 20 to 24-25 mg/dL per unit (making each unit of insulin "stronger" in your calculations, so you use less correction insulin). This represents approximately a 20-25% adjustment.

The rationale: Farxiga enhances insulin sensitivity through multiple mechanisms—reducing glucotoxicity, promoting weight loss, and decreasing insulin resistance. Research demonstrates that dapagliflozin added to insulin therapy significantly improved pancreatic beta-cell function (HOMA-beta) and reduced insulin requirements 4. This means your existing insulin will work more effectively.

Insulin-to-Carbohydrate Ratio (ICR) Adjustment

Change your ICR from 1:5 to 1:6 (meaning 1 unit covers 6 grams of carbohydrate instead of 5). This is approximately a 20% reduction in mealtime insulin.

This adjustment prevents postprandial hypoglycemia since Farxiga will be lowering your glucose throughout the day, including after meals. Studies show that SGLT2 inhibitors reduce postprandial glucose excursions and time above range 4, 5.

Monitoring and Titration Strategy

Critical first 2-4 weeks:

• Check fasting glucose daily to guide basal insulin adjustments
• Monitor pre-meal and 2-hour post-meal glucose to assess ICR adequacy
• Watch for hypoglycemia, especially overnight (Farxiga increases nocturnal hypoglycemia risk when combined with insulin 5)

Adjustment algorithm:

• If fasting glucose consistently <70 mg/dL: Reduce Lantus by additional 2-4 units
• If fasting glucose 70-100 mg/dL: Current dose appropriate
• If fasting glucose >130 mg/dL after 1 week: Increase Lantus by 2 units every 3 days 1

For correction doses and meal coverage:

• If experiencing hypoglycemia <70 mg/dL: Further liberalize ISF and ICR by 10-15%
• If glucose remains elevated: Adjust more conservatively

Important Safety Considerations

Common pitfalls to avoid:

1. Don't wait to reduce insulin: The FDA label shows no significant pharmacokinetic interaction between dapagliflozin and insulin 6, but the pharmacodynamic effect (glucose lowering) is additive and immediate. Reduce insulin on day 1 of Farxiga.

2. Hypoglycemia risk is real: Studies show similar overall hypoglycemia rates, but nocturnal hypoglycemia specifically increases 2, 5. The 2025 ADA guidelines emphasize that for hypoglycemia without clear cause, lower the corresponding insulin dose by 10-20% 1.

3. Ensure adequate hydration: SGLT2 inhibitors cause osmotic diuresis. Dehydration can worsen if combined with hypoglycemia.

4. Ketone monitoring: Although rare with type 2 diabetes, be aware of euglycemic DKA risk, especially if you become ill or significantly reduce carbohydrate intake.

Expected Outcomes

Research demonstrates that adding dapagliflozin to insulin therapy typically results in:

• HbA1c reduction of 0.4-0.6% beyond insulin alone 4, 3
• Weight loss of 1-2 kg over 3-6 months 2, 3
• Improved time in range (3.9-10 mmol/L) by approximately 6% 5
• Reduced glycemic variability 4, 7

These benefits occur while using less insulin, which is why proactive dose reduction is essential to prevent hypoglycemia and maintain quality of life.