GLP-1 Peptides: The Complete Guide to Semaglutide, Tirzepatide & Beyond

GLP-1 receptor agonists are the most significant pharmacological development in obesity medicine in decades. They have transformed weight management from a willpower narrative into a neurobiological one — and they represent the only peptide category with overwhelming RCT evidence, FDA approval, and cardiovascular outcome data.

This guide covers everything from established drugs to the next-generation pipeline to the compounded GLP-1 debate.

The GLP-1 mechanism: why these drugs work

GLP-1 (glucagon-like peptide-1) is an incretin hormone released from intestinal L-cells after eating. Its natural functions include:

• Appetite suppression: GLP-1 acts on hypothalamic appetite centers, reducing hunger and increasing satiety
• Delayed gastric emptying: Food moves through the stomach more slowly, prolonging fullness
• Glucose-dependent insulin secretion: Enhances insulin release only when glucose is elevated — low hypoglycemia risk
• Glucagon suppression: Reduces hepatic glucose output
• Direct cardiovascular benefits: GLP-1 receptors exist on cardiomyocytes and vascular endothelium

Natural GLP-1 has a half-life of 2–3 minutes. Pharmaceutical GLP-1 agonists are modified for extended duration — from hours (exenatide) to a full week (semaglutide, tirzepatide).

The current approved landscape

Semaglutide (Ozempic / Wegovy / Rybelsus)

The reference standard GLP-1 agonist.

Key data:

• STEP 1: 16.9% body weight loss at 68 weeks (2.4 mg weekly)
• SELECT: 20% reduction in major adverse cardiovascular events (MACE) — the first obesity drug to demonstrate CV mortality benefit
• STEP 5: Weight loss maintained through 104 weeks of continued treatment

Formulations: Weekly subcutaneous injection (Ozempic 0.25–2 mg for T2D; Wegovy 0.25–2.4 mg for obesity) and daily oral (Rybelsus 3–14 mg for T2D).

Dose escalation: 0.25 mg → 0.5 mg → 1 mg → 1.7 mg → 2.4 mg (Wegovy), each step held for 4 weeks minimum.

Tirzepatide (Mounjaro / Zepbound)

The first dual GLP-1/GIP agonist, with the largest weight-loss effect sizes in RCT history.

Key data:

• SURMOUNT-1: 22.5% body weight loss at 72 weeks (15 mg)
• SURMOUNT-5 (head-to-head vs. semaglutide): Tirzepatide produced ~5% greater weight loss than semaglutide at maximum doses
• SURPASS (T2D trials): Superior HbA1c reduction versus semaglutide

The GIP question: GIP (glucose-dependent insulinotropic polypeptide) was previously considered obesogenic — its inclusion seemed paradoxical. The current understanding is that pharmacological GIP receptor activation at these doses produces different effects than physiological GIP — enhanced fat oxidation, improved insulin sensitivity, and potentially better lean mass preservation.

Dose escalation: 2.5 mg → 5 mg → 7.5 mg → 10 mg → 12.5 mg → 15 mg weekly, each step held for 4 weeks.

Liraglutide (Saxenda / Victoza)

The first-generation daily GLP-1 agonist. Still relevant but largely superseded by weekly options.

Key data: ~8% body weight loss at 56 weeks (Saxenda 3 mg daily). Less effective than semaglutide and tirzepatide, but with a longer safety track record.

Current role: An option for patients who prefer daily dosing, cannot tolerate weekly formulations, or where insurance covers Saxenda but not Wegovy/Zepbound.

The next-generation pipeline

Retatrutide (triple agonist — GLP-1/GIP/glucagon)

Phase 2 data showed 24.2% weight loss at 48 weeks — exceeding tirzepatide. The glucagon receptor component adds hepatic fat mobilization and thermogenesis. Phase 3 trials ongoing; anticipated approval 2026–2027.

Cagrilintide (amylin analog)

A long-acting amylin analog being studied in combination with semaglutide (CagriSema). Amylin is a pancreatic hormone co-released with insulin that promotes satiety through hindbrain pathways distinct from GLP-1. The combination targets two independent appetite-suppression pathways.

CagriSema phase 2 data: Up to 15.6% weight loss at 32 weeks — promising as a combination that may reduce the lean mass loss issue by targeting different satiety pathways.

Orforglipron (oral non-peptide GLP-1)

A small-molecule GLP-1 agonist (not a peptide) with once-daily oral dosing. Phase 3 trials showing ~14% weight loss. The significance is accessibility — an oral pill that doesn't require injection could dramatically expand treatment adoption.

The critical issues

Muscle loss

The single biggest clinical concern with GLP-1 agonists. Approximately 25–40% of weight lost is lean mass — an unacceptable ratio for long-term metabolic health.

Mitigation strategies:

• Resistance training 3–4× per week — non-negotiable
• Protein intake ≥1.6 g/kg/day — GLP-1 agonists suppress appetite broadly, including for protein. Deliberate protein targeting is essential
• Lower doses — faster weight loss correlates with more lean mass loss. The minimum effective dose produces better body composition outcomes than maximum dosing
• CagriSema and dual-pathway approaches may improve body composition partitioning, though data is early

Weight regain after discontinuation

SURMOUNT-4 demonstrated that patients who discontinued tirzepatide after 36 weeks regained two-thirds of lost weight within 52 weeks. STEP 1 extension data showed similar regain patterns with semaglutide. This is not a failure of the drugs — it reflects the neurobiology of obesity. The hypothalamic setpoint reverts when the pharmacological signal is removed.

Implication: Current evidence supports long-term or indefinite treatment for sustained weight management. This has cost, access, and philosophical implications that the field is still navigating.

Compounded GLP-1s

With brand-name supply constraints and high costs ($1,000+/month without insurance), compounded semaglutide from 503B pharmacies has become widespread.

Key considerations:

• Compounded semaglutide uses the semaglutide salt form (typically semaglutide sodium or semaglutide acetate), not the base form in Novo Nordisk's products
• Quality varies by pharmacy — sterility, potency, and stability testing should be verified
• FDA has taken enforcement actions against some compounders; regulatory status is fluid
• Cost is substantially lower ($100–300/month vs. $1,000+)
• The active molecule is the same, but formulation differences may affect bioavailability and side effect profile

GI side effects

Nausea (40–44%), diarrhea (30%), vomiting (24%), and constipation (24%) are common, particularly during dose escalation. Most are transient and resolve with continued use. Slow escalation, smaller meals, and avoiding lying down after eating help. Persistent severe GI symptoms warrant dose reduction.

Pancreatitis risk

A rare but serious adverse effect. The absolute risk increase is small (0.1–0.2%), but any severe epigastric pain radiating to the back requires immediate evaluation and drug discontinuation.

Who should consider GLP-1 therapy

• BMI ≥30 or BMI ≥27 with weight-related comorbidity
• Failed prior attempts with diet, exercise, and behavioral modification
• Willing to commit to long-term treatment (this is not a short-term fix)
• Able to implement resistance training and high-protein nutrition alongside medication
• Under physician supervision with regular metabolic monitoring

Who should not

• BMI <27 without medical indication — GLP-1 agonists for cosmetic weight loss in normal-weight individuals is off-label, carries unnecessary risk, and contributes to supply constraints for indicated patients
• Personal or family history of medullary thyroid carcinoma or MEN2 syndrome
• Active pancreatitis or severe GI disease
• Pregnancy or planning pregnancy within the treatment period
• History of eating disorders — GLP-1 agonist appetite suppression can worsen disordered eating patterns