GLP-1 Agonists Explained: Semaglutide, Tirzepatide, Retatrutide & Beyond

Glucagon-like peptide 1 (GLP-1) is a 30-amino-acid incretin hormone that plays a central role in the body's metabolic regulation. It is produced by enteroendocrine L-cells in the distal small intestine and colon, primarily in response to nutrient ingestion.

When food enters the gastrointestinal tract, GLP-1 is released into the bloodstream and exerts several coordinated effects: it stimulates glucose-dependent insulin secretion from pancreatic beta cells, suppresses glucagon release from alpha cells, slows gastric emptying, and promotes satiety through receptors in the hypothalamus and brainstem. These actions collectively help regulate blood sugar levels and energy balance.

Natural GLP-1 has a major limitation: it is rapidly degraded by the enzyme dipeptidyl peptidase-4 (DPP-4), giving it a plasma half-life of only 1–2 minutes. This short half-life means natural GLP-1 cannot be used therapeutically. The breakthrough that launched the GLP-1 agonist revolution was the development of synthetic analogues resistant to DPP-4 degradation, enabling half-lives measured in hours or even days.

The clinical impact of GLP-1 receptor agonists has been extraordinary. Originally developed for type 2 diabetes, they have proven to be highly effective for weight management, and emerging research suggests benefits for cardiovascular disease, non-alcoholic steatohepatitis (NASH/MASH), chronic kidney disease, and possibly neurodegenerative conditions.

Semaglutide is the most widely prescribed GLP-1 receptor agonist and arguably the most impactful peptide drug of the 2020s. Developed by Novo Nordisk, it is marketed as Ozempic (for type 2 diabetes) and Wegovy (for chronic weight management), with an oral formulation available as Rybelsus.

Semaglutide is a modified version of human GLP-1 with two key structural changes. First, an amino acid substitution at position 8 (alanine to alpha-aminoisobutyric acid) makes it resistant to DPP-4 degradation. Second, a C-18 fatty diacid chain is attached via a linker at position 26, enabling the peptide to bind to serum albumin. This albumin binding dramatically extends its half-life to approximately 165 hours (roughly 7 days), allowing once-weekly dosing.

Clinical trial data has been striking. In the STEP trials for weight management, semaglutide 2.4 mg weekly produced mean weight loss of approximately 15–17% of body weight over 68 weeks, with some participants achieving over 20% reduction. For diabetes, the SUSTAIN trials demonstrated superior HbA1c reductions compared to most other glucose-lowering therapies.

Beyond metabolic effects, the SELECT trial demonstrated that semaglutide reduced major adverse cardiovascular events (MACE) by 20% in overweight or obese individuals with established cardiovascular disease — a landmark finding that expanded the therapeutic rationale well beyond glycaemic control. Common side effects include nausea, vomiting, diarrhoea, and constipation, which are typically most pronounced during dose titration and tend to diminish over time.

Tirzepatide represents the next evolution in incretin therapy. Developed by Eli Lilly, it is marketed as Mounjaro and acts as a dual GIP/GLP-1 receptor agonist — simultaneously activating both the glucose-dependent insulinotropic polypeptide (GIP) and the GLP-1 receptor.

GIP is another incretin hormone, released from K-cells in the upper small intestine in response to nutrient intake. Like GLP-1, it stimulates insulin secretion and is rapidly degraded by DPP-4. However, GIP and GLP-1 act through distinct receptors and have partially non-overlapping biological effects. GIP receptors are highly expressed in adipose tissue, where GIP signalling may influence fat storage, lipolysis, and energy expenditure.

The rationale behind tirzepatide is that dual agonism produces additive or synergistic effects beyond what either pathway achieves alone. Clinical data supports this hypothesis. In the SURMOUNT-1 trial, tirzepatide at the highest dose (15 mg weekly) produced mean weight loss of approximately 22.5% over 72 weeks — significantly exceeding semaglutide's results in comparable populations. HbA1c reductions in the SURPASS trials were similarly impressive, with many participants achieving normal glucose levels.

Tirzepatide is structured as a 39-amino-acid linear peptide based on the GIP sequence but engineered to also activate GLP-1 receptors. Like semaglutide, it incorporates a fatty acid moiety for albumin binding and extended half-life (approximately 5 days), supporting once-weekly injection. The side-effect profile is broadly similar to semaglutide, with gastrointestinal symptoms being the most common adverse events.

Retatrutide (LY3437943), developed by Eli Lilly, is a triple agonist that targets three receptors simultaneously: GLP-1, GIP, and the glucagon receptor. The addition of glucagon receptor agonism introduces a fundamentally new metabolic lever.

Glucagon, often viewed simply as the "opposite of insulin," has important effects on energy expenditure and hepatic fat metabolism. Activation of the glucagon receptor increases resting metabolic rate, stimulates hepatic lipid oxidation, and promotes thermogenesis. While chronic glucagon excess would be detrimental (raising blood glucose), the concurrent GLP-1 and GIP agonism in retatrutide counterbalances the hyperglycaemic effect while harnessing the metabolic benefits.

Phase 2 clinical trial data for retatrutide was remarkable. At the highest dose tested (12 mg weekly), participants achieved mean weight loss of approximately 24.2% at 48 weeks, with some individuals losing over 30% of their body weight. Perhaps more significantly, a substantial proportion of participants with metabolic dysfunction-associated steatotic liver disease (MASLD) achieved complete resolution of liver steatosis, suggesting that the glucagon component provides particular benefit for hepatic fat clearance.

Other triple or multi-agonist candidates in development include survodutide (a dual GLP-1/glucagon agonist being developed by Boehringer Ingelheim, which has shown strong efficacy in MASH trials) and various investigational compounds targeting combinations of GLP-1, GIP, glucagon, and amylin receptors. The field is moving rapidly toward precision polypharmacology — single molecules that address multiple metabolic pathways simultaneously.

Comparing the major incretin-based therapies requires consideration of efficacy, mechanism, dosing, and tolerability. Below is a summary of the key differences.

Semaglutide (single GLP-1 agonist): - Targets: GLP-1 receptor only - Dosing: Once weekly (injectable) or once daily (oral) - Mean weight loss: ~15–17% (68 weeks) - HbA1c reduction: ~1.5–1.8% - Notable: Cardiovascular outcome benefit demonstrated (SELECT trial); oral formulation available - Approved: Yes (UK, EU, US) for type 2 diabetes and weight management

Tirzepatide (dual GIP/GLP-1 agonist): - Targets: GIP and GLP-1 receptors - Dosing: Once weekly (injectable) - Mean weight loss: ~20–22.5% (72 weeks) - HbA1c reduction: ~2.0–2.5% - Notable: Superior weight loss versus semaglutide in head-to-head trial - Approved: Yes (UK, EU, US) for type 2 diabetes; weight management approvals progressing

Retatrutide (triple GLP-1/GIP/glucagon agonist): - Targets: GLP-1, GIP, and glucagon receptors - Dosing: Once weekly (injectable) - Mean weight loss: ~24% (48 weeks, Phase 2) - Notable: Strongest liver fat reduction; Phase 3 trials ongoing - Approved: Not yet — Phase 3 clinical trials in progress

Cagrilintide + Semaglutide (CagriSema): - Targets: Amylin and GLP-1 receptors (fixed-dose combination) - Dosing: Once weekly (injectable) - Mean weight loss: Phase 3 data emerging; expected to exceed semaglutide monotherapy - Notable: Amylin-mediated satiety enhancement; Novo Nordisk development - Approved: Not yet — Phase 3 clinical trials in progress

Each generation of incretin therapy appears to deliver incrementally greater metabolic benefits, though long-term safety data for newer agents is still accumulating.

Access to GLP-1 agonists in the United Kingdom has evolved rapidly, shaped by both regulatory approvals and ongoing supply constraints.

Semaglutide is available in the UK both as Ozempic (for type 2 diabetes, NHS-funded subject to NICE guidelines) and Wegovy (for weight management, approved by MHRA). NHS access to Wegovy for weight management has been subject to phased rollout and supply limitations. Private prescriptions are available through licensed weight management clinics and pharmacies, though costs can be substantial — typically £150–300 per month depending on the dose.

Tirzepatide (Mounjaro) received MHRA approval for type 2 diabetes and has been progressing through NICE appraisal for NHS funding. Its availability for weight management in the UK has been expanding through both NHS and private channels. As with semaglutide, supply pressures have periodically affected availability.

Retatrutide, survodutide, and CagriSema are not yet available in the UK outside of clinical trials. Participants in ongoing Phase 3 studies at UK trial sites may have access, but general availability is likely several years away pending regulatory review.

It is critically important to note that purchasing GLP-1 agonists from unregulated online sources carries significant risks, including counterfeit products, incorrect dosing, and contamination. All GLP-1 agonists are prescription-only medicines in the UK and should only be obtained through legitimate medical channels. The information in this article is educational and should not be used to self-prescribe or self-treat any medical condition.

The incretin therapy landscape is evolving at an extraordinary pace, with several major developments expected over the coming years.

Oral formulations represent a key frontier. Oral semaglutide (Rybelsus) has demonstrated that peptide-based GLP-1 agonists can be delivered by mouth, though its current bioavailability (approximately 1%) requires relatively high doses and strict fasting protocols. Next-generation oral formulations using improved absorption technologies — including SNAC-based enhancers, permeation-enhancing carriers, and ionic liquid formulations — may achieve significantly better oral bioavailability, potentially making daily injections obsolete for many patients.

Longer-acting formulations are also in development. Compounds with monthly or even less frequent dosing could improve adherence and convenience. Subcutaneous implants and depot formulations that release GLP-1 agonists over weeks or months are being explored by several pharmaceutical companies.

Combination approaches beyond CagriSema are being investigated. Researchers are exploring combinations with myostatin inhibitors (to preserve muscle mass during weight loss), GDF15 analogues (to enhance satiety through different pathways), and FGF21 analogues (to improve metabolic flexibility). The goal is to achieve not just weight loss but metabolically healthy body composition change — reducing fat while preserving or even building lean mass.

Finally, the potential applications of GLP-1 pathway modulation continue to expand. Clinical trials are investigating incretin-based therapies for Alzheimer's disease (based on GLP-1 receptor expression in the brain), polycystic kidney disease, substance use disorders (via reward pathway modulation), and obstructive sleep apnoea. The versatility of GLP-1 signalling suggests that these peptides may become foundational tools in medicine well beyond their metabolic origins.

All content in this article is for educational purposes only and does not constitute medical advice. GLP-1 agonists are prescription medicines with specific indications and contraindications. Consult a qualified healthcare professional for personalised medical guidance.