Evidence review

What Is Actually in a Compounded GLP-1: Additives, Salt Forms and a Chemistry Problem

Many compounders add B12, B6, niacinamide, glycine or carnitine to differentiate their products. None has proven benefit for weight loss, the combinations are largely unt

Why this question is rarely asked

Almost every comparison in this category treats compounded semaglutide and compounded tirzepatide as single products with a price attached. They are not. What arrives in the vial depends on which compounder made it, what was added, and which chemical form of the drug was used — and those three variables are invisible from a pricing page.

This page is about what is in the preparation, which is the question that determines whether any of the trial evidence quoted for these drugs applies to what you are buying.

1. The additives

Compounders commonly add extra ingredients to differentiate their preparations from the FDA-approved products. The most common is vitamin B12. Others include B6, niacinamide, glycine and carnitine.

Two things follow. None of these has proven benefit for weight loss or diabetes management, so the addition is a marketing differentiator rather than a clinical one. And the resulting combinations are largely untested in clinical trials — there is no trial of tirzepatide-plus-B12, because that combination is not a product anybody has developed.

The usual defence is that B12 is safe and widely supplemented. That is true of B12 taken as a supplement. It is a different claim from B12 being inert when co-formulated in a sterile injectable alongside a peptide, which is what the next section is about.

2. The chemistry finding

A 2026 study found that when tirzepatide is compounded with B12, the two substances can chemically bond — forming a new molecule that is not present in the FDA-approved drug.

That is a materially different situation from two ingredients sharing a vial. A conjugate is a new chemical entity. Its potency, its stability, its absorption and its safety profile are properties of that new entity, not of tirzepatide, and none of them has been characterised.

It also means the milligram figure on the label may not describe how much unbound tirzepatide is actually present. A patient prescribed a dose in milligrams is being dosed on an assumption that the substance in the vial is the substance in the trials.

We are flagging this prominently because it is a specific, checkable, recently published finding that almost no comparison site in this category has covered — and it bears directly on the central question of whether compounded preparations can be treated as equivalent to the approved product.

3. Salt forms

The approved products contain specific chemical structures tested in the trial programmes. Some compounders use chemically distinct salt forms instead — semaglutide sodium rather than semaglutide, for example.

These salt forms have not been proven safe and effective in humans and are not FDA-approved. Effectiveness can differ because of the salt form difference; that is not a technicality about nomenclature, it is a statement that the substance is not the same substance.

The FDA has issued warning letters over salt-form products, and the distinction has been one of the clearer lines in enforcement.

What this does to the trial evidence

Every efficacy figure quoted across this market — roughly 20.9% at tirzepatide 15 mg in SURMOUNT-1, 13.7% for semaglutide 2.4 mg in the SURMOUNT-5 head-to-head — comes from trials of the approved product, at studied doses, in the approved formulation.

Those figures do not transfer to a preparation that contains an additive, a conjugate, or a different salt. That is not a pedantic point about regulatory categories. It is the practical reason a compounded product cannot be described as equivalent, and it is what the FDA warning letters have been about.

Five questions to put to any compounded provider

1. Which pharmacy compounds this, and is it registered as a 503A pharmacy or a 503B outsourcing facility? No provider in our dataset has answered this for us.
2. Does the preparation contain anything besides the active ingredient and standard excipients? Ask for the full formulation, not a marketing description.
3. Which chemical form is used — the base molecule, or a salt?
4. Can I see a certificate of analysis matched to the batch number on my vial?
5. What is the beyond-use date, and what were the storage conditions?

A provider that answers all five plainly is one you can assess. A provider that treats the formulation as proprietary is asking you to accept an unspecified substance on trust.

What we are not saying

That compounded preparations are unsafe as a class. Lawfully compounded medicine prepared by a licensed pharmacy against a valid prescription is an established part of practice, and there are real reasons patients use it.

What we are saying is that "compounded tirzepatide" names a category, not a product; that the differences inside that category are material rather than cosmetic; and that a price comparison which ignores formulation is comparing things that may not be comparable.

Questions readers actually ask