Salt forms: semaglutide acetate vs free acid
The FDA-approved Ozempic and Wegovy use a specific salt form of semaglutide. Compounded versions can use that same form or a different one — and the FDA has explicitly warned that the salt forms are not equivalent for clinical use. The chemistry behind this isn't academic; it affects what's actually in your vial.
What "salt form" means
Many drug molecules contain acidic or basic groups that can be neutralized with a counter-ion to form a salt. The salt and the free acid (or free base) are chemically distinct preparations of the same active molecule, with different physical properties:
- Solubility in water and other diluents.
- Stability at various temperatures and pH conditions.
- Crystal form, which affects bioavailability and shelf life.
- Manufacturing reproducibility.
The molecule does the same thing biologically once dissolved (it binds the GLP-1 receptor either way). But getting it to the bloodstream reliably requires consistent salt-form chemistry.
Semaglutide's specifics
Ozempic and Wegovy use semaglutide as a free acid (or sodium salt — Novo's exact formulation is subject to patent details). The formulation is buffered with phosphate and stabilized to a specific pH where solubility, stability, and absorption are all optimized.
Compounding pharmacies typically can't access the FDA-approved salt-form preparation directly — that's patented. They source bulk API ("active pharmaceutical ingredient") that may be:
- Semaglutide acetate: the most common compounded form, derived from a different synthesis path.
- Semaglutide free acid (research grade): used by some vendors; not the same chemistry as the FDA-approved version.
- Semaglutide sodium: alternative salt form, sometimes used.
The FDA's 2023–2024 statements
FDA explicitly noted in compounding-bulks evaluations that semaglutide acetate is "not the same drug" as the FDA-approved Ozempic/Wegovy semaglutide. This was part of the basis for excluding compounded semaglutide from the 503B bulks list (see FDA 2026 decision).
The agency's point: even when both products contain "semaglutide" by name, the manufacturing path, salt form, impurity profile, and pharmacokinetic behavior can differ in clinically meaningful ways.
What this could mean in practice
For a patient injecting compounded semaglutide:
- Effect at the same labeled dose may differ from Ozempic. If the salt form has slightly different solubility, the absorbed mass per labeled mg differs.
- Stability over the in-use window may differ. Salt form affects how quickly the drug degrades in solution.
- Impurity profile differs. Different synthesis paths produce different process impurities, with different potential immunogenicity.
Reading the COA
A reputable compounding pharmacy's Certificate of Analysis should specify:
- The exact salt form used — "semaglutide acetate," "semaglutide free acid," "semaglutide sodium," etc.
- The API source — manufacturer name and lot number.
- Purity ≥98%, with HPLC chromatogram.
- Impurity profile — what the other ≤2% is.
- Identity confirmation — mass spectrometry showing the molecular weight matches.
A COA that doesn't specify the salt form, or that lists only "semaglutide" without further detail, is incomplete. The compounding pharmacy should be able to clarify on request.
What this means for choice
- FDA-approved products are the only fully-characterized option. Ozempic, Wegovy, Mounjaro, Zepbound — all use validated salt forms.
- Legitimate 503A compounded products with documented salt forms and complete COAs are next-best.
- Grey-market "research use only" peptide typically doesn't disclose salt form. Treat the salt-form question as unanswerable for these products.
FAQ
Does using semaglutide acetate instead of Ozempic cause harm?
Not necessarily. The salt form alone is unlikely to be dangerous. The risks come from impurities and concentration variations that often accompany non-FDA-approved sources, not from the salt form itself.
Why does compounded semaglutide work at all if the salt form differs?
Both forms contain the same biological molecule. Once dissolved and absorbed, semaglutide acetate and semaglutide free acid both bind the GLP-1 receptor. The differences are in how reliably you get it into circulation, not in what it does once there.
Are tirzepatide salt forms also a concern?
Yes, similar logic applies. Compounded tirzepatide may use different salt forms than Mounjaro/Zepbound. FDA has made similar statements about tirzepatide non-equivalence.
Can I find out which salt form my pharmacy uses?
Ask. A legitimate compounding pharmacy will tell you. The label may not specify; the COA should. If neither does and the pharmacy can't answer, that's a quality-control signal.
Related reading
Document salt form per vial
Peptide Protocol stores salt form and COA per vial. Quality questions get a clean answer instead of guesswork.
Get the iPhone app →Informational and educational only. Not medical advice. Consult a licensed clinician before starting, changing, or stopping any peptide protocol. Mentions of investigational, compounded, or research-use peptides are for informational purposes; many such substances are not FDA-approved for human use.