GLP-1s for Weight Loss: What You Are Actually Buying
This is the one category on this site where the drugs demonstrably work and the trials are enormous. Which is exactly why the risk has moved off the molecule and into the vial.
Who this is for
You want to lose weight. You have seen three names, probably in this order: semaglutide, tirzepatide, retatrutide. You are deciding between a prescription and a vial from a website, and you have already priced both, which is the real reason you are still reading. Nothing here tells you which to do.
What people actually run
Reporting what circulates. This is observation, and none of these numbers came from a trial unless I say so.
What differs underneath those three names is the supply route, not the biology.
| Route | What is known about it |
|---|---|
| One. A prescription, which semaglutide and tirzepatide both have | Has narrowed since the compounding window opened by the shortages closed |
| Two. A clinic or telehealth service selling a compounded version | Among 75 weight loss clinics and medspas in West Virginia and Oklahoma trading in late 2025, 56% offered GLP-1 products combined with B vitamins (PMID 42467450) |
| Three. A vial of powder and a bottle of bacteriostatic water | For retatrutide it is the only route that exists. An unreviewed preprint analysing Reddit discussion to December 2025 counted 13,589 people reporting current use |
The conventions along that third route are worth naming precisely, because they are conventions and nothing more. For tirzepatide, a microdose means any weekly amount below the 2.5 mg the label starts at, and those figures come off vendor and telehealth pages, which are marketing rather than evidence. For retatrutide the charts teach a mixing recipe and a target to work up to, none of which any trial used, for a compound approved in no country. The circulating numbers are vial arithmetic, not trial arithmetic.
What each one is actually built on
Semaglutide. The human evidence runs past 25,000 people in outcome trials alone. In STEP 1, 1,961 people lost 14.9% of body weight against 2.4% on placebo, and SELECT followed 17,604 people with obesity and heart disease and cut heart attacks, strokes and cardiovascular deaths by 20%. (PMID 37952131) Two things get left out: the effect is about a third smaller with type 2 diabetes, and in the withdrawal trial the group that switched to placebo gained back 6.9%.
Tirzepatide. Sold as Mounjaro and Zepbound. SURMOUNT-1 ran 72 weeks and produced 20.9% weight loss at 15 mg against 3.1% on placebo (PMID 35658024), and SURMOUNT-5 put it head to head against semaglutide and got 20.2% against 13.7%. The lowest dose ever carried through a randomised trial with a sustained weight endpoint was 1 mg, and it produced 0.9 kg over 26 weeks in people with type 2 diabetes.
Retatrutide. A weekly injection acting on three receptor systems instead of one. Phase 2 gave 24.2% weight loss at 12 mg over 48 weeks against 2.1% on placebo (PMID 37366315), and the phase 3 TRIUMPH-1 readout in 2,335 participants reported 28.3% at 80 weeks, or 25.0% on the more conservative estimand. Those phase 3 results were announced by press release and have not been peer reviewed.
How it compares
Two more names sit on the same shelf. The class lines up like this.
| Approved? | What the evidence behind it covers | |
|---|---|---|
| Semaglutide | Yes, as Ozempic and Wegovy | Weight, and hard endpoints: heart attacks, strokes, kidney failure |
| Tirzepatide | Yes, as Mounjaro and Zepbound | Weight, blood sugar, and sleep apnoea on the label |
| Retatrutide | No, in any country | Weight and blood sugar. Phase 2 peer reviewed, phase 3 by press release |
| Mazdutide | In China only, twice, since 2025 | Weight in Chinese adults. FDA treats it as an unapproved new drug |
| Cagrilintide | No, alone or in combination | Weight, and every human study of it has the same sponsor |
Cagrilintide is the odd one out. It is a copy of amylin, a different hormone, and most of the percentages printed under its name were earned by a pen that also holds semaglutide.
The honest read
The evidence here is not thin, and pretending otherwise would be its own kind of dishonesty. The risk sits in three distinctions instead.
Approved is not a formality, and retatrutide is approved nowhere. For semaglutide and tirzepatide, going grey market is a substitution: a characterised, inspected, assayed version of the same molecule exists, and a doctor can hand it to you. For retatrutide no such version exists at any price. Whatever arrives is the only one you will ever see, and no reference sample exists to tell you it is wrong.
The one published test of what is actually sold found two errors pointing opposite ways. Researchers bought semaglutide from three online sellers in 2024 and had it analysed. Purity came back at 14.37, 8.97 and 7.7 percent against 99 percent advertised. And the amount of actual semaglutide in those vials ran 28.6 to 38.7 percent higher than the label claimed. (PMID 39509151)
Most people picture the grey market failure as a watered down drink. What that lab found was a glass mostly made of something other than alcohol that still held more alcohol than the bottle promised. The one who poured carefully to the line got hit hardest. The researchers' own stated concern was overdose, not underdosing.
The only vial level result published for retatrutide points the same way. A vendor commissioned test on a BulkGLP 10 mg vial reported content of 12.05 mg against the 10 mg label, 121% of stated. I could find no peer reviewed analysis of grey market retatrutide.
Two of these three have a fabricated trial registration. On 31 July 2026, Morgan McSweeney published an investigation into a cluster of eight ClinicalTrials.gov registrations under one sponsor, Hudson Biotech, all marked recruiting, all at a single site. (Dr Noc) Three of the eight say in their own text that they are fictional examples. The two that concern this page do not. NCT07481747 reproduces SURMOUNT-1's official title character for character and carries Eli Lilly's own internal protocol code for that trial with "(b)" appended. NCT07467447 copies the title of Lilly's phase 2 retatrutide study word for word and calls itself recruiting, when the study it copied completed on 22 November 2022. Semaglutide has nothing of the kind attached to it.
And one thing is true of all three. The weight comes back when you stop. That is what the withdrawal arms of the manufacturers' own trials found, so it is not a course you finish.
If you are going to do this anyway
This is the part I actually care about, and almost none of it is about the molecules. Two things decide how this goes: where the vial came from, and arithmetic.
Before the money leaves
Demand a certificate of analysis tied to your batch, not to the product
In April 2025 the United States International Trade Commission barred importation of products containing tirzepatide and of "products purporting to contain tirzepatide." That last phrase is a federal agency recording, in an enforcement order, that some of what ships may contain no drug at all.
Read the content line, sometimes called assay, and not only the purity line
Content is how many milligrams are actually in the vial, a different question from purity. Both published results above failed on content, in the same direction, upward.
Check the vial size on your own label against the vial size the vendor chart assumes
Sizes vary between vendors, and for retatrutide 10, 12, 24, 30 and 60 mg all circulate. The same reconstitution instruction poured into a different vial gives a different concentration, so the same units on the syringe deliver a different dose.
How to read a certificate, on real ones including an annotated retatrutide one.
The arithmetic is the other half, and it is worse here than in most categories. The one published case report of harm is a man who bought retatrutide online, escalated from 10 mg to 20 mg weekly, then injected again the next day after forgetting the first, roughly 40 mg across two days. He had a week of intractable diarrhoea, dehydration and acute kidney injury, and recovered. The full walkthrough of the math is here.
What would make you stop
Gastrointestinal effects, which are dose related rather than rare
In the retatrutide phase 3, nausea ran 42.4% at the top dose against 14.8% on placebo, and more than one in nine people stopped that arm for adverse events.
The boxed warning for thyroid C-cell tumours that both approved drugs carry
Seen in rats. Human relevance undetermined.
Pancreatitis, bowel obstruction and gastroparesis, all raised signals in the semaglutide safety comparison
The pancreatitis estimate carries a margin of error wide enough to be genuinely uncertain.
Two things never make it onto a checklist, because there is no check to run. Sterility and endotoxin you cannot assess at home at any price. All three of those semaglutide samples carried bacterial toxin, and nothing about their appearance would have told you. If you compete under an anti-doping code, semaglutide is banned by WADA, and retatrutide falls into the category covering substances approved nowhere, prohibited at all times.
My take
I have spent most of this project arguing that the confidence around these compounds is manufactured. Here it is not. Somebody ran the trials, they were large, and they published the unflattering results too.
The evidence is real and it is being used as cover for something it does not cover. A clinic quotes SURMOUNT-1's 20.9% while selling a compounded product no trial ever used. The number is true. It is just not a number about the thing in the box.
Retatrutide is the sharpest version, holding the best results and the worst position at once.
The question I would ask myself is not which molecule. It is whether I am buying the drug that was tested, or something that shares its name.
Frequently asked
Is retatrutide approved anywhere?
It is approved in no country, and Lilly, the company that makes it, says it cannot legally be sold or marketed for human use. Every vial in circulation therefore came from outside the proper supply chain.
Which of the three produced the most weight loss in trials?
On the headline figures, retatrutide. Its phase 2 trial reported 24.2% weight loss at the top dose over 48 weeks against 2.1% on placebo, and the phase 3 TRIUMPH-1 readout in 2,335 participants reported 28.3% at 80 weeks. Those phase 3 results were announced by press release and have not been peer reviewed.
Is compounded or grey market semaglutide the same as the prescription version?
In the one published test of what is actually sold, researchers bought semaglutide from three online sellers in 2024 and had it analysed. Purity came back at 14.37, 8.97 and 7.7 percent against 99 percent advertised, and those vials held 28.6 to 38.7 percent more actual semaglutide than the label claimed. The researchers' stated concern was overdose.
Does the weight come back when you stop?
That is what the withdrawal arms of the manufacturers' own trials found, and it is true of all three, so it is not a course you finish. In the semaglutide withdrawal trial, the group that switched to placebo gained back 6.9%.
What side effects were reported in the trials?
The gastrointestinal effects are dose related rather than rare. In the retatrutide phase 3, nausea ran 42.4% at the top dose against 14.8% on placebo, and more than one in nine people stopped that arm for adverse events. Both approved drugs carry a boxed warning for thyroid C-cell tumours seen in rats.
You have priced both, so I would like the two numbers: what the prescription actually costs you per month after your insurance, and what the vial costs delivered. The gap people quote in forums is wider than the gap most people face, and nobody publishes the real one.
This content is for educational and informational purposes only and is not medical advice. Peptides discussed are research compounds and may not be approved for human use. Nothing here should be used to diagnose, treat, cure, or prevent any disease. Always consult a qualified healthcare professional before starting any peptide, supplement, or protocol. Individual responses vary. Do not self-administer compounds without proper medical supervision.
Registrations not to cite
- NCT07481747
Efficacy and Safety of Tirzepatide Once Weekly Versus Placebo in Participants Who Are Either Obese or Overweight With Weight-Related Comorbidities (SURMOUNT-1)
The registry lists it as recruiting, Hudson Biotech, first posted 19 March 2026.
A copy of the real SURMOUNT-1 trial, word for word in its title, down to the participant count. The file number gives it away: I8F-MC-GPHK(b) where Eli Lilly's genuine number has no letter on the end.
- NCT07467447
Effect of LY3437943 Versus Placebo in Participants Who Have Obesity or Are Overweight
The registry lists it as recruiting, Hudson Biotech, first posted 12 March 2026.
A registry clone. Copies a real Eli Lilly retatrutide trial under the sponsor Hudson Biotech at a single hospital in Shenzhen.
Registry details are from ClinicalTrials.gov, the United States government trial registry, as it read on 14 September 2026.
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