The science behind compounded GLP-1 medications: a clinical guide
What a GLP-1 receptor agonist does, how semaglutide and tirzepatide differ, and what “compounded” actually means in the US. Including the clarification the FDA publishes about 503A pharmacies.
Compounded medications are not FDA-approved: the agency does not review their safety, effectiveness or quality before they reach a patient, and it says so in those words. What is regulated is how they are made and how they are prescribed: a 503A pharmacy, an individualised prescription and a prior clinical evaluation. This article explains what the molecule does in the body, how the two active ingredients in use today differ, and what the word “compounded” actually means — which is not the same as “approved”.
If you want to know how the clinical programme works, it is set out in the medical weight loss protocol with GLP-1. What follows is the science behind it, with sources.
What a GLP-1 receptor agonist does
GLP-1 is an incretin hormone the gut releases after eating. Its synthetic version — a peptide that binds the same receptor — does three things described in the mechanism literature:
- A satiety signal in the central nervous system. There are GLP-1 receptors in the hypothalamus and the brainstem, and that signal takes part in appetite regulation. This is where the popular term “food noise” comes from: it describes the reduction in the urge to eat that patients report.
- Slower gastric emptying. Food stays longer in the stomach, which prolongs the feeling of fullness.
- A glucose-dependent effect on blood sugar. Insulin secretion rises when glucose is present, not indiscriminately.
Drucker reviewed these mechanisms in *Cell Metabolism* in 2018, with 141 supporting references. That is the source I use here, and it is the field's reference review.
Semaglutide and tirzepatide are not the same molecule
It is worth being precise, because the two are confused daily. Semaglutide is a GLP-1 receptor agonist. Tirzepatide is a dual agonist: it acts on GIP and GLP-1 receptors at once. That does not make one better or worse by definition; it makes them different, and the choice between them is an individual clinical decision.
The trial most often cited to compare them is SURMOUNT-5, published in *The New England Journal of Medicine* in 2025 (DOI `10.1056/NEJMoa2416394`). What it did, and its limits, matter as much as the result:
| Study feature | Detail |
|---|---|
| Design | Phase 3b, open-label (not double-blind), 751 participants with obesity and without type 2 diabetes |
| Duration | 72 weeks, maximum tolerated dose in both groups |
| Primary result | Mean weight change: −20.2% with tirzepatide versus −13.7% with semaglutide |
| Funding | Eli Lilly, the maker of tirzepatide |
That last detail does not invalidate the study, but it is published: it is a trial funded by the maker of one of the two drugs being compared. Being open-label — patients knew what they were taking — is a recognised limitation in trials of this kind.
In reference clinical trials, participants achieved significant weight reductions alongside dietary and physical activity support. Individual results vary according to each person's biological response. A trial's figures describe a group average, not what will happen to one person. For the mechanism in more detail, see what GLP-1 is and how it works.
What “compounded” means, and what it does not
This is where confusion is most common, and where precision matters.
In the United States, compounding is governed by section 503A of the Federal Food, Drug, and Cosmetic Act. The FDA describes the conditions of that section as follows:
“Section 503A describes the conditions under which compounded human drug products are exempt from the FD&C Act sections on FDA approval prior to marketing, current good manufacturing practice (CGMP) requirements, and labeling with adequate directions for use.”
In other words: a 503A pharmacy is not subject to CGMP. The facilities that are subject to it are 503B, the voluntary *outsourcing facility* category created in 2013. The distinction is not a technicality: if a website states that 503A pharmacies comply with CGMP, it is misdescribing the legal regime that applies to its own medicines.
And the second clarification, published by the FDA on the same page:
“Compounded drugs are not FDA-approved. This means that FDA does not review these drugs to evaluate their safety, effectiveness, or quality before they reach patients.”
So the correct language is not “FDA-approved compounded medication”. It is this, and it is what we use:
The base active ingredient holds FDA approval in its brand-name reference medicines. Personalised compounded formulations are prepared and dispensed by state-licensed 503A pharmacies under strict individualised medical prescription following a clinical consultation.
What the law does require of a 503A pharmacy is what makes the model work: a valid prescription, individualised for a specific patient. A 503A pharmacy cannot manufacture in series or dispense without that prescription.
How it is prescribed, and why the evaluation is not a formality
The medication is not chosen from a catalogue. A licensed professional prescribes it after assessing that person's history, contraindications, concurrent medication and metabolic goals. That is why the first step in any serious protocol is a consultation, and why you will not find a dose, a titration schedule or an adjustment protocol in this article: that requires a clinical act and a professional to perform it.
State coverage is set out in territorial coverage: 49 states, Washington D.C. and Puerto Rico.
What this article does not say, and not by omission
- No doses. Not starting, not titrating, not maintaining.
- No promises of results. No kilos, no timelines, no percentages presented as an individual expectation.
- We do not compare brand-name medicines by name. A 503A compound is not an approved equivalent of any commercial product.
- We do not sell. What patients engage at Nectalis is the evaluation and the clinical follow-up; the medication is dispensed by a 503A pharmacy under prescription.
What you can do while on a protocol is eat well, and there the nutritional support is real: the cottage cheese bowl with pear, chia and pistachio provides 31.8 g of protein and 8.6 g of fibre per serving, with macros calculated from USDA data.
Frequently asked questions
Is a compounded medication FDA-approved?
No. The FDA does not review compounded medications for safety, effectiveness or quality before they reach patients. What is approved is the base active ingredient in its brand-name reference medicines.
Is it the same as a brand-name medicine?
No. A brand-name medicine goes through the FDA approval process as a finished product. A 503A compounded formulation is prepared and dispensed individually, under prescription, and is not an approved equivalent of any commercial product.
Do I need a prescription?
Yes. It is a prescription medicine, and a 503A pharmacy can only dispense it against an individualised prescription issued by a licensed professional after a clinical consultation.
How much weight will I lose?
That question is not answered by an article, and be wary of the one that tries. Trials publish mean changes in groups of hundreds of people, with dietary and physical activity support. Individual response varies. What we can tell you is how the process works: it starts with a clinical evaluation.
In which states is it available?
In 49 states — all but California — plus Washington D.C. and Puerto Rico. Details are on the territorial coverage page.
Sources
- FDA — Human Drug Compounding Laws. `https://www.fda.gov/drugs/human-drug-compounding/compounding-laws-and-policies`. Source of the quotes on 503A, CGMP, 503B and the non-approval of compounded drugs. Verified 2026-10-05.
- SURMOUNT-5 team — *Tirzepatide as Compared with Semaglutide for the Treatment of Obesity*. N. Engl. J. Med. 2025. DOI `10.1056/NEJMoa2416394`. PMID 40353578. Design, results and funding taken from the abstract indexed in Europe PMC.
- Drucker, D. J. — *Mechanisms of Action and Therapeutic Application of Glucagon-like Peptide-1*. Cell Metab. 2018. DOI `10.1016/j.cmet.2018.03.001`. Mechanism review, 141 references.
- NIDDK (NIH) — Prescription Medications to Treat Overweight & Obesity. `https://www.niddk.nih.gov/health-information/weight-management/prescription-medications-treat-overweight-obesity`. Institutional backing that these treatments are prescription medication under clinical supervision. Verified 200.
- MANUAL_COMPLIANCE_EDITORIAL_SEO.md (Nectalis, internal). Source of the literal 503A declaration reproduced in this article.
Medical Disclaimer: *The information provided in this article is for educational and informational purposes only and does not constitute medical advice, diagnosis, or treatment. Never disregard professional medical advice or delay in seeking it because of something you have read on this website. Always consult your physician before making any changes to your medication or lifestyle regimen.*
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