Approach to GLP-1 Analogue Management in Obesity

1. Background and Mechanism

• Pancreatic effects: Stimulate glucose-dependent insulin secretion; inhibit glucagon secretion
• Gastrointestinal: Slow gastric emptying, prolonging satiety
• Central (hypothalamic): Activate GLP-1 receptors in the arcuate nucleus and nucleus tractus solitarius, suppressing appetite and reducing food intake
• Combined GIP/GLP-1 (tirzepatide): GIP receptor agonism in the brain acts synergistically with GLP-1 receptor activation to enhance weight loss

• Harrison's Principles of Internal Medicine 22E, p. 3236

2. Approved Agents for Obesity (Indications)

• Sabiston Textbook of Surgery, p. 3543; Harrison's, p. 3236-3237

3. Patient Selection - Who to Start

• BMI ≥ 30 kg/m² (obesity), OR
• BMI ≥ 27 kg/m² with at least one weight-related comorbidity (T2DM, hypertension, dyslipidemia, OSA, CVD)
• Failed lifestyle intervention (diet + exercise counseling)

• T2DM - any GLP-1 RA (dual benefit)
• Established CVD - semaglutide 2.4 mg preferred (SELECT trial: 20% reduction in MACE)
• Heart failure with preserved EF - semaglutide (STEP-HFpEF trial benefit)
• Pre-bariatric surgery bridging - use to reduce operative risk in very high BMI
• Post-bariatric weight regain - semaglutide especially effective (~10% additional loss)

• Harrison's, p. 3236-3237; Sabiston, p. 3547

4. Dose Titration Protocol

Liraglutide

• Start: 0.6 mg SC once daily x 1 week
• Increase by 0.6 mg weekly
• Target: 3.0 mg SC daily
• Titration period: ~5 weeks

Semaglutide (Wegovy)

• Start: 0.25 mg SC once weekly x 4 weeks
• Escalate in 4-week increments: 0.5 mg → 1.0 mg → 1.7 mg → 2.4 mg
• Target: 2.4 mg SC weekly
• Full titration: ~17 weeks

Tirzepatide

• Start: 2.5 mg SC once weekly x 4 weeks
• Increase by 2.5 mg every 4 weeks
• Target: 5 mg, 10 mg, or 15 mg SC weekly (max 15 mg)
• Adjust upward based on tolerability

• Yamada's Textbook of Gastroenterology, p. 2149; Harrison's, p. 3237

5. Efficacy - Trial Data

FIGURE: One-year mean weight loss for anti-obesity medications compared to placebo. Lira=liraglutide; Sema=semaglutide; TZP=tirzepatide. Harrison's Principles of Internal Medicine 22E.

• Harrison's, p. 3236-3237; Sabiston, p. 3545

6. Contraindications

• Personal or family history of medullary thyroid carcinoma (MTC)
• Multiple Endocrine Neoplasia type 2 (MEN2)
• Pregnancy (teratogenic risk; discontinue before conception attempt)
• Known hypersensitivity to the agent

• History of pancreatitis (use with caution)
• Severe gastroparesis (GI slowing may worsen)
• Renal impairment (monitor for dehydration from GI side effects)
• Pre-operative context: increased aspiration risk - typically held for 1 week (liraglutide daily) or 2-4 weeks (semaglutide/tirzepatide weekly) before elective surgery per perioperative guidelines

• Harrison's, p. 3236-3237; Miller's Anesthesia, p. 1179-1184

7. Adverse Effects and Monitoring

• Nausea, vomiting, diarrhea, constipation (generally mild and transient; peak during dose escalation)
• Injection site reactions

• Acute pancreatitis (monitor for abdominal pain; discontinue if suspected)
• Gallbladder disease / cholelithiasis (rapid weight loss increases risk)
• Tachycardia (mild, usually transient)
• Eustachian tube dysfunction (emerging adverse event signal from recent systematic review, PMID 39666743)

• Weight / BMI at each visit
• Blood glucose (hypoglycemia risk when combined with insulin/sulfonylurea)
• HbA1c if diabetic
• Renal function if GI side effects significant (dehydration)
• Thyroid (clinical vigilance; routine calcitonin not universally recommended)

8. Assessing Response and Adjusting Therapy

• If < 5% weight loss after 12-16 weeks at the target dose → consider switching agent or augmenting with lifestyle/behavioural therapy
• If ≥ 5% weight loss → continue; reassess comorbidity improvement
• Reassess at 16 weeks; if inadequate response, consider:
  • Escalating to higher dose tier (e.g., tirzepatide 10 → 15 mg)
  • Adding complementary medication (e.g., phentermine/topiramate or naltrexone/bupropion if not already on)
  • Referral for bariatric surgery evaluation (if BMI criteria met)

9. Special Populations

10. Treatment Algorithm Summary

Obesity (BMI ≥30 or ≥27 + comorbidity)
         ↓
Lifestyle intervention (diet + exercise + behavioural therapy)
         ↓ Insufficient response
Pharmacotherapy: GLP-1 RA (first choice in most patients)
         ↓
Choose agent based on comorbidities:
  - CVD/HFpEF → Semaglutide 2.4 mg weekly
  - T2DM → Any GLP-1 RA; tirzepatide dual benefit
  - General obesity → Tirzepatide (greatest efficacy) or semaglutide
         ↓
Titrate slowly to target dose (4-17 weeks per agent)
         ↓
Assess response at 12-16 weeks at target dose
  ≥5% loss → Continue, monitor
  <5% loss → Reassess adherence, switch/add agent, or refer for bariatric surgery
         ↓
Long-term continuation (weight regains if stopped)

Recent Evidence Update