ThirdPartyVerified
Compound Guides/Semaglutide

Semaglutide

GLP-1 Receptor Agonist

Updated September 2026
Regulatory notice

Semaglutide is an FDA-approved prescription medication (Ozempic, Wegovy, Rybelsus). Unlike most compounds in our guides, semaglutide has completed extensive human clinical trials and is available as a pharmaceutical product. The regulatory landscape for compounded versions is evolving rapidly. This guide covers the science and lab testing — not dosing, prescribing, or sourcing.

Quick Facts

Class
GLP-1 Receptor Agonist
Sequence Length
31 amino acids (modified)
Molecular Weight
~4,113.58 Da
Molecular Formula
C187H291N45O59
CAS Number
910463-68-2
Developer
Novo Nordisk
FDA Approvals
Ozempic (2017, T2D), Wegovy (2021, weight), Rybelsus (2019, oral T2D), Oral Wegovy (2025, oral weight)
Human Clinical Data
Extensive — multiple Phase III trials (STEP, SUSTAIN, PIONEER programs)

1. What Is Semaglutide

Semaglutide is a GLP-1 (glucagon-like peptide-1) receptor agonist — a synthetic peptide designed to mimic the natural incretin hormone GLP-1 that the body releases after eating. It is a 31-amino-acid peptide with two key structural modifications that distinguish it from natural GLP-1:

Position 8 (GLP-1 numbering): Alanine → Aib (alpha-aminoisobutyric acid). This is residue 2 of the semaglutide peptide itself; the FDA label numbers it position 8 using native GLP-1 numbering. The substitution makes the peptide resistant to degradation by the enzyme DPP-IV, which rapidly breaks down natural GLP-1 in the body (natural GLP-1 has a half-life of roughly 1–2 minutes).
Lysine 26: Fatty acid side chain. A C18 fatty diacid (octadecanedioic acid) is attached to the lysine at position 26 through a hydrophilic spacer, and position 34 is changed from lysine to arginine so only one chain attaches. The fatty acid lets semaglutide bind albumin in the blood, extending its elimination half-life to approximately one week — enabling once-weekly dosing.

These modifications transform a peptide with a 2-minute half-life into one with a ~7-day half-life, which is what makes semaglutide clinically practical as a once-weekly injection.

2. Mechanism of Action

Appetite and Satiety

Semaglutide acts on GLP-1 receptors in the brain (hypothalamus and brainstem) to reduce appetite and increase feelings of fullness. This central mechanism is the primary driver of the weight loss observed in clinical trials.

Insulin Secretion

GLP-1 receptor activation in the pancreas stimulates glucose-dependent insulin release. “Glucose-dependent” is a key distinction — semaglutide enhances insulin secretion only when blood glucose is elevated, which reduces the risk of hypoglycemia compared to some other diabetes medications.

Gastric Emptying

Semaglutide slows the rate at which food leaves the stomach, contributing to prolonged feelings of fullness. This also explains common gastrointestinal side effects (nausea, constipation) particularly during dose escalation.

Glucagon Suppression

GLP-1 receptor activation suppresses glucagon secretion from pancreatic alpha cells, further contributing to blood glucose control.

3. Clinical Evidence

Unlike most peptides in our compound guides, semaglutide has an extensive body of human clinical trial data from multiple Phase III programs:

SUSTAIN trials (Type 2 Diabetes). A series of randomized controlled trials demonstrating semaglutide's efficacy in reducing HbA1c and body weight in patients with type 2 diabetes, leading to FDA approval of Ozempic in 2017.
STEP trials (Weight Management). Clinical trials showing mean weight loss of approximately 15–16% over 68 weeks with semaglutide 2.4 mg (STEP 1 reported 14.9%), leading to FDA approval of Wegovy in 2021.
STEP UP (higher dose). FDA approved a 7.2 mg dose strength — labelled Wegovy HD, a new strength within the existing Wegovy label rather than a separate product — on 19 March 2026, for adults who have tolerated 2.4 mg and need further weight reduction. In the STEP UP trial at 72 weeks, the FDA label reports 18.8% mean weight loss at 7.2 mg versus 15.5% at 2.4 mg and 3.9% on placebo; the published on-treatment estimate reaches 20.7%. Altered skin sensation was more common at 7.2 mg (around 22%) than at 2.4 mg (around 6%).
SELECT trial (Cardiovascular). Demonstrated a 20% relative reduction in major adverse cardiovascular events (6.5% versus 8.0% of participants, an absolute difference of about 1.5 percentage points) in overweight or obese adults with established cardiovascular disease.
PIONEER trials (Oral Semaglutide). Trials supporting the development of Rybelsus, the oral tablet formulation of semaglutide for type 2 diabetes.

This level of clinical evidence is qualitatively different from the preclinical animal data available for most research peptides. Semaglutide has well-characterized efficacy and safety profiles from thousands of participants across multiple Phase III trials.

4. Pharmaceutical vs. Compounded Semaglutide

This is the most important distinction for anyone evaluating semaglutide quality:

FDA-Approved (Ozempic, Wegovy, Rybelsus)

Manufactured by Novo Nordisk under cGMP conditions. Tested in Phase III clinical trials with thousands of participants. Prescribed by physicians with established dosing protocols. Subject to full FDA post-market surveillance.

Compounded Semaglutide

Prepared by compounding pharmacies using semaglutide base or sodium salt. Not FDA-approved as a finished product. Quality depends on the individual compounding facility's processes and testing. Regulatory status is actively changing.

Evolving regulatory landscape (as of September 2026)

Large-scale compounding of a drug is generally permitted only while FDA lists it as being in shortage. Novo Nordisk resolved its manufacturing shortages, FDA removed semaglutide from the shortage list, and on 27 August 2026 the Fifth Circuit affirmed that removal — eliminating the shortage-based legal basis for mass compounding. Alongside that, FDA announced warning letters to 30 telehealth companies over illegal marketing of compounded GLP-1 products on 3 March 2026, followed by 25 more in June, and on 30 April 2026 proposed excluding semaglutide, tirzepatide and liraglutide from the 503B outsourcing facility bulk drug substance list. That exclusion is still a proposal, not a final rule. What remains is narrow, patient-specific 503A compounding with a documented clinical need; copies of the approved drug are not permitted. This area keeps changing, so check current FDA guidance rather than relying on this page.

Why this matters for lab testing: The quality assurance question is fundamentally different for these two categories. FDA-approved semaglutide has undergone manufacturing validation, stability testing, and regulatory review. Compounded semaglutide's quality depends entirely on the compounding facility and whether they test their product. This is where third-party lab testing becomes critical — a COA from an independent lab is the primary way to evaluate compounded product quality.

5. Purity and Lab Testing

HPLC Purity

Semaglutide is a complex 31-amino-acid peptide with a fatty acid modification. High-quality semaglutide should test at 95% or above via HPLC. The fatty acid side chain and linker add complexity to synthesis compared to simpler peptides.

Mass Spectrometry

The expected molecular weight is approximately 4,113.58 Da. A purity test alone cannot tell you the vial holds semaglutide rather than another peptide, so mass spec is what confirms identity. What has actually been documented in the grey market is worth knowing: a peer-reviewed 2024 analysis of three no-prescription online vendors claiming 99% purity measured 7.7%, 9.0% and 14.4% purity, found 28–39% more semaglutide than labelled, and detected bacterial endotoxin in every sample. FDA has separately warned that compounders have used semaglutide sodium and acetate salt forms, which are different active ingredients from the approved drug, and has seized counterfeit Ozempic pens — one of which contained insulin glargine instead of semaglutide.

Semaglutide Base vs. Semaglutide Sodium

Compounded semaglutide may be prepared from semaglutide base or semaglutide sodium salt. The sodium salt form has different solubility characteristics. The COA should specify which form was tested, as this affects molecular weight calculations and concentration measurements.

Peptide Content

For compounded semaglutide products, net peptide content testing is particularly important. This measures the actual concentration of active semaglutide in the finished product — a critical check for compounded injectables where the labeled dose must match what's in the vial.

6. What to Look For in a Semaglutide COA

HPLC purity of 95% or higher from a named, independent third-party lab

Mass spectrometry confirming molecular weight near 4,113.58 Da — this confirms the vial holds semaglutide and not another peptide

Specification of semaglutide base vs. sodium salt form

Net peptide content matching the labeled concentration (for finished products)

Endotoxin testing results (for injectable preparations)

Batch or lot number matching the product received

Lab reference number verifiable with the testing laboratory

For a complete guide to evaluating COAs, see How to Read a Peptide COA.

Summary

Semaglutide is unique among peptides covered in our guides: it is an FDA-approved prescription medication with extensive Phase III clinical trial data supporting its efficacy for type 2 diabetes and weight management. The quality question centers on compounded versions, where third-party lab testing — particularly mass spectrometry to confirm identity, plus purity and endotoxin testing — is the primary safeguard, given the purity failures and contamination documented in grey-market product. The regulatory landscape is actively changing and large-scale compounding no longer has a shortage-based legal basis; check current FDA guidance before making sourcing decisions.