Glp1 Insulin Su Dose Reduction

GLP-1 RA + Insulin / Sulfonylurea — Proactive Dose Reduction

Purpose

Adding a GLP-1 RA to a patient already on a sulfonylurea or basal insulin increases hypoglycemia risk. The monographs mandate "consider a reduction" — this skill turns that into a concrete starting number and a monitoring plan.

Activation

Trigger when:

• Starting a GLP-1 RA in a patient on SU and/or basal insulin.
• Escalating the GLP-1 RA dose in the same combination.
• Planning insulin withdrawal as glycemic control improves (see scenarios).
• Patient reports hypoglycemia after adding/escalating GLP-1 RA.

Baseline Hypoglycemia Numbers to Calibrate Against

• SURPASS-5 (tirzepatide + basal insulin glargine): baseline glargine reduced by 20% when HbA1c ≤ 8.0% at randomisation.
• SURPASS-4 (tirzepatide + glargine + SU background): severe hypo 0.5–1.1% on Mounjaro vs 2.4% on glargine alone; hypo < 3.0 mmol/L 9.9–13.8% on Mounjaro vs 21.6% on glargine alone.
• Wegovy STEP trials: hypoglycemia rare without concomitant SU/insulin.

Starting Rules

Sulfonylurea

• Reduce SU by 50% at GLP-1 RA start if HbA1c < 8.0%.
• Reduce SU by 25–50% if HbA1c 8.0–9.0%.
• Maintain SU if HbA1c > 9.0% (less urgent).
• Consider stopping SU altogether once GLP-1 RA is at maintenance and glycemia improved — SU is replaceable with safer agents (SGLT2i, metformin).
• Hypoglycemia with SU often outlasts a dose drop — glipizide / glimepiride half-lives are long.

Basal Insulin

• Reduce basal insulin by 20% at GLP-1 RA start if HbA1c ≤ 8.0% (per SURPASS-5 protocol).
• Reduce basal insulin by 10–20% if HbA1c 8.0–10.0%.
• Do not reduce if HbA1c > 10% or significantly hyperglycemic; let titration drive reduction.
• Titrate further based on self-monitored fasting glucose targets.

Prandial / Bolus Insulin

• Monograph note (Mounjaro): not studied with short/medium-acting insulin or dual formulations.
• In practice, if patient is on bolus insulin and GLP-1 RA is added (off-label combination), reduce bolus doses alongside basal.

Monitoring

• Self-monitor blood glucose daily minimum during first 2 weeks and after each GLP-1 RA escalation.
• Tell patient to recognise hypo symptoms and carry fast-acting carbohydrate.
• Reassess at 2 weeks, 4 weeks, and each dose escalation.
• Consider CGM if available — underestimated hypo risk in SU-treated patients.

Insulin Withdrawal Protocol (from published scenarios)

In stable, well-controlled patients on basal insulin who have reached GLP-1 RA maintenance:

• Reduce basal insulin gradually as glycemic improvement becomes evident.
• Withdraw insulin entirely when dose is < 15 IU/day and glycemia remains controlled (cardiorenal-protected patient benefits from metabolic simplification).

Drugs That Do NOT Usually Cause Hypoglycemia

When GLP-1 RA is combined with these, no pre-emptive reduction is needed:

• Metformin
• SGLT2 inhibitors
• DPP-4 inhibitors (avoid — do not combine with GLP-1 RA)
• Thiazolidinediones

(Though always re-assess at each visit.)

Rules & Constraints

1. Always reduce the riskier agent first — SU is higher-risk than basal insulin for severe hypo.
2. Don't wait for hypo to happen — pre-emptive reduction is the standard.
3. SU half-life matters — reducing doesn't remove overnight hypoglycemia risk for ~24 h.
4. No dose reduction for metformin/SGLT2i at GLP-1 RA initiation (unless GI AEs — see glp1-gi-ae-pharmacological-rescue for the metformin-diarrhoea interaction).
5. In T2D, Wegovy — discontinuation can increase blood glucose; monitor glucose when stopping too, not only when starting.
6. Pediatric Wegovy + T2D — safety data limited; specialist input recommended.

Reference

• MOUNJARO Product Monograph, §4.1 Dosing Considerations, §7 Endocrine and Metabolism, §8 Adverse Reactions (Hypoglycemia tables).
• WEGOVY Product Monograph, §4.2 Patients with T2D, §7 Endocrine and Metabolism.
• SURPASS-5 protocol (baseline insulin 20% reduction at HbA1c ≤ 8.0%).
• Gorgojo-Martínez JJ et al. J Clin Med 2023;12:145 (published scenario 4 for insulin withdrawal).