Semaglutide. Tirzepatide. Liraglutide. The GLP-1 class went from obscure diabetes drugs to the most-discussed medications in modern medicine in roughly five years. Here's what they actually do, what the published trial data shows, and why "compounded GLP-1" is a different conversation from the brand-name versions.
What GLP-1 is, biologically
GLP-1 stands for glucagon-like peptide-1. It's a peptide your gut naturally releases after you eat. Its job: tell your pancreas to release insulin, slow down how quickly your stomach empties, and signal your brain that you're full.
Native GLP-1 has a half-life of just a few minutes — your body breaks it down quickly. So pharmaceutical chemists figured out how to build modified versions that mimic GLP-1 but stick around longer in the bloodstream. These modified peptides are called GLP-1 receptor agonists.
How GLP-1 receptor agonists work
An "agonist" is a molecule that binds to a receptor and triggers the same response the natural molecule would. GLP-1 receptor agonists bind to GLP-1 receptors throughout your body — particularly in the pancreas, gut, and brain — and produce a longer-lasting version of the natural GLP-1 effect.
The downstream consequences include:
- Slower gastric emptying. Food stays in your stomach longer, so you feel full longer.
- Increased insulin response. When your blood sugar rises, the pancreas responds more strongly.
- Reduced appetite signaling. Brain pathways that drive food cravings get modulated.
- Reduced glucagon output. Your liver releases less glucose between meals.
These effects together can lead to a shift in metabolic patterns and changes in body composition over months — well-documented in the published clinical trial literature.
Mono-receptor vs dual-receptor agonists
Not all GLP-1 medications are the same. The category split into two important branches:
Mono-receptor agonists (GLP-1 only)
Semaglutide is the most-studied molecule in this class. It binds only to the GLP-1 receptor. FDA-approved as Ozempic (for type 2 diabetes) and Wegovy (for chronic weight management). Manufactured by Novo Nordisk. Once-weekly subcutaneous injection.
Other mono-receptor GLP-1 agonists include Liraglutide (Saxenda, Victoza), Dulaglutide (Trulicity), and several older agents.
Dual-receptor agonists (GLP-1 + GIP)
Tirzepatide is a newer molecule that binds to two incretin receptors — GLP-1 and GIP (glucose-dependent insulinotropic polypeptide). FDA-approved as Mounjaro (type 2 diabetes) and Zepbound (chronic weight management). Manufactured by Eli Lilly. Once-weekly subcutaneous injection.
The dual-receptor mechanism produces somewhat different metabolic effects than GLP-1-alone, and the published trial data shows larger mean body weight changes for Tirzepatide compared to Semaglutide.
What the trials actually show
STEP 1 — Semaglutide
Published in the New England Journal of Medicine in 2021, the STEP 1 trial randomized 1,961 adults with overweight or obesity to receive Semaglutide 2.4 mg weekly or placebo, alongside lifestyle intervention. Over 68 weeks, the Semaglutide group showed a mean body weight reduction of ~14.9%, compared to ~2.4% in the placebo group. Wilding et al., NEJM 2021.
SURMOUNT-1 — Tirzepatide
SURMOUNT-1 was the corresponding trial for Tirzepatide, published in NEJM in 2022. Across three Tirzepatide doses (5, 10, and 15 mg weekly), mean body weight reduction at 72 weeks ranged from ~15% to ~20.9%. The 15 mg group showed the largest effect. Jastreboff et al., NEJM 2022.
These trial results are population means under controlled conditions with structured lifestyle intervention and study-protocol adherence. Individual outcomes in real-world clinical use vary substantially based on dose, adherence, baseline metabolic health, lifestyle, and other factors. The trial numbers are a reference point for what's possible — not a representation of what any individual patient will experience.
Compounded vs FDA-approved versions
The brand-name FDA-approved versions of these medications (Ozempic, Wegovy, Mounjaro, Zepbound) are made by their respective manufacturers under FDA-approved processes.
Compounded versions are different. A 503A compounding pharmacy prepares a patient-specific formulation of Semaglutide or Tirzepatide pursuant to a valid prescription, when a licensed prescriber determines an FDA-approved drug isn't appropriate for the patient's specific needs.
A few important things to understand about compounded GLP-1:
- Compounded medications are not FDA-approved. They are prepared on a patient-specific basis. This is legal under federal compounding regulations.
- The regulatory landscape has been moving fast. FDA removed Tirzepatide from the drug-shortage list in October 2024 and Semaglutide in February 2025, which has affected what 503A pharmacies can compound and how. Anyone considering compounded GLP-1 should understand the current status.
- Source quality matters. Verify your physician is working with a 503A-licensed US pharmacy with state-board oversight. Lior works exclusively with licensed 503A pharmacies and is transparent about which compounds come from which source.
Side effects and clinical considerations
GLP-1 receptor agonists have a documented side-effect profile. The most common (per the published trials and FDA labeling):
- Gastrointestinal effects (nausea, vomiting, diarrhea, constipation), most common in the first weeks and during dose titration
- Reduced appetite (often desired but worth noting)
- Possible interactions with other medications and certain pre-existing conditions
- Specific contraindications including history of medullary thyroid carcinoma and MEN-2 syndrome (per FDA labeling)
Baseline lab work (kidney function, liver function, thyroid markers, A1C) is generally clinically appropriate before starting a GLP-1 protocol. A licensed physician determines individually what's required for your specific case.
Where Lior fits in
Lior Health offers GLP-1 receptor agonist therapy through our Body Composition protocol. Like every Lior protocol, it's prescribed by a US-licensed physician for the individual patient and custom-compounded by a 503A pharmacy.
Pre-prescribing labs are required for our Body Composition protocol — we believe baseline metabolic data is the right standard of care for this category, even where federal law may not strictly require it. Off-label use is prescribed at the physician's judgment based on individual assessment.
If GLP-1 therapy isn't right for you, your physician will tell you. That filtering is part of what you're getting.