Potency Testing for Compounded GLP-1s: Which Labs Do It and What They Find

When a compounding pharmacy labels a vial as "semaglutide 5 mg/mL," is there actually 5 mg of semaglutide in every milliliter? Potency testing answers that question — and the answer isn't always what the label promises.

This article covers how potency testing works for compounded GLP-1 preparations, what independent testing has revealed about accuracy across the industry, and what the results mean for patients who are titrating based on the assumption that their dose is what the label says it is.

What Potency Testing Measures

Potency testing (also called assay testing) determines the actual concentration of active drug in a compounded preparation. For compounded semaglutide, this means measuring how many milligrams of semaglutide peptide are present per milliliter of solution — and comparing that measured value against the labeled claim.

The standard analytical method is HPLC (High-Performance Liquid Chromatography), which separates the components of a solution and quantifies the semaglutide peak against a reference standard of known concentration. The result is reported as a percentage of the labeled claim — ideally between 90% and 110%, which is the typical USP acceptance range for compounded preparations.

What the Numbers Mean

• 100% of label claim: The vial contains exactly the amount of semaglutide stated on the label. This is the target.
• 90-110%: Within the standard acceptance range. A result of 95% means the vial has 4.75 mg/mL instead of the labeled 5 mg/mL — a clinically insignificant difference for most patients.
• Below 90%: Underdosed. A result of 80% means you're getting only 4 mg/mL when you think you're getting 5 mg/mL. At standard titration doses, this 20% deficit could reduce efficacy and slow your treatment response.
• Above 110%: Overdosed. A result of 120% means 6 mg/mL instead of 5 mg/mL. This increases the risk of dose-dependent side effects — particularly nausea, which is directly correlated with semaglutide dose.

Who Performs the Testing

Potency testing for compounded preparations is performed by analytical testing laboratories — some of which specialize in compounding pharmacy quality testing. These labs are separate from the compounding pharmacy and provide independent, third-party verification.

Laboratories that test compounded preparations should hold appropriate accreditations: ISO 17025 (general laboratory competence), or state laboratory licenses relevant to pharmaceutical testing. Some labs also participate in proficiency testing programs where their analytical capabilities are independently verified against reference standards.

Types of Testing Programs

• Release testing: The pharmacy tests each batch (or a representative sample from each batch) before releasing it for dispensing. This is the gold standard — every patient receives a product that has been verified for potency.
• Periodic verification testing: The pharmacy tests representative batches on a defined schedule (e.g., monthly or quarterly) rather than every batch. This is less comprehensive but still provides ongoing quality data.
• Stability testing: Products are tested at timed intervals (day 0, 30, 60, 90) to verify that potency is maintained throughout the beyond-use dating period.
• Complaint-driven testing: Testing is performed only when a problem is reported. This reactive approach provides no assurance about the majority of products that go undisputed.

What Independent Testing Has Found

Several independent analyses of compounded GLP-1 preparations have been published or reported since 2023, revealing a wide range of quality outcomes:

Some preparations tested within the 90-110% acceptance range, confirming accurate dosing and competent compounding practices. Others fell significantly outside this range — with some samples testing as low as 60-70% of labeled potency, and others exceeding 130%. These extreme outliers suggest either serious compounding errors (incorrect weighing, calculation mistakes, or use of degraded API) or deliberate underdosing to reduce costs.

The pattern that emerges from available testing data is that pharmacy quality is highly variable. PCAB-accredited pharmacies and those with documented quality management systems tend to produce more consistent results. Unaccredited pharmacies, particularly those operating at high volume with rapid turnaround, show wider potency variation.

What Patients Can Do

• Ask your provider: "Does the pharmacy perform potency testing on compounded GLP-1 preparations? Can they share the results?" A pharmacy that tests and shares results is a pharmacy that has nothing to hide.
• Request the CoA: A certificate of analysis for a finished product batch should include an assay result showing the measured potency as a percentage of the labeled claim.
• Watch for clinical signs of underdosing: If you experience a sudden loss of appetite suppression, increased food noise, or stalled weight loss mid-titration — and you haven't made changes to diet or activity — potency variation is worth considering as a variable.
• Compare across pharmacies: If you switch providers and experience a noticeable change in medication effects at the same labeled dose, that difference may reflect potency variation between the two pharmacies' products.

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