Safety
GLP-1 Dosing Errors: Why Concentration Changes Are Dangerous
Units measure volume, not drug. When concentration changes, the math changes.
FDA has documented dosing errors with compounded GLP-1 injectables, several tied to varying concentrations and confusion between "units" and milligrams. This piece explains why concentration changes are dangerous and how to avoid the specific mistakes regulators have flagged.
Why units are not the same as drug
Insulin-style syringes measure volume in "units," not milligrams of medication. The amount of drug a given number of units delivers depends entirely on the concentration of the vial. If a compounded semaglutide vial's concentration changes between shipments — which can happen when a pharmacy reformulates or when you switch products — the same unit count that was correct before can now deliver a very different, potentially dangerous, dose. FDA has specifically documented overdoses arising from exactly this units-versus-milligrams confusion with compounded GLP-1 products.
The specific errors regulators have flagged
The documented error patterns include: patients drawing the same number of units after a concentration change, effectively over- or under-dosing; confusion when instructions are given in milligrams but the syringe reads in units; and errors self-titrating without clear per-dose volume instructions. These are not exotic edge cases — they're predictable consequences of a system where the drug amount depends on a concentration the patient may not know has changed.
How to avoid them
Follow the prescription label as the single source of truth for each vial, and re-read it every shipment rather than reusing a remembered unit count. If a new vial's concentration differs from the last, the correct number of units changes — do not carry over the old number. When instructions are in milligrams, confirm exactly how many units that is for this vial's concentration. And never estimate: if the per-dose volume isn't crystal clear, contact the prescriber or pharmacy before injecting.
| Vial | Concentration | Units drawn | Drug delivered |
|---|---|---|---|
| Old vial | 2.5 mg/mL | 20 units | 0.5 mg |
| New vial (higher concentration) | 5 mg/mL | 20 units | 1.0 mg (double) |
The illustration shows the mechanism: identical units, doubled concentration, doubled dose. This is why a concentration change is the specific event to watch for, and why the label — not habit — must govern each injection.
Primary sources
This article's regulatory and safety statements draw on the following primary sources from the U.S. Food and Drug Administration:
- FDA — FDA's Concerns with Unapproved GLP-1 Drugs Used for Weight Loss
- FDA — Understanding the Risks of Compounded Drugs
Frequently asked questions
Why can the same units be a different dose?
Because units measure volume, not medication. The drug delivered equals volume times concentration. If concentration changes between vials and you draw the same units, the dose changes proportionally. This is the mechanism behind the overdoses FDA has documented with compounded GLP-1 products.
What should I do if a new vial looks different?
Stop and check the label and concentration before injecting. If the concentration differs from your previous vial, the correct unit count is different — don't reuse the old number. Contact the pharmacy or prescriber to confirm the exact per-dose volume for the new vial. This is a clinical safety step, not a convenience one.
Is this a reason to avoid compounded injectables?
It's a reason to handle them carefully and to value providers who give clear, per-vial dosing instructions and consistent concentrations. The error is preventable with label discipline. But the risk is real and documented, which is why dosing clarity is a legitimate factor when choosing a provider — and why the medication decision belongs with a prescriber.
Evidence status. Pricing figures are provider-reported or independently calculated as of 2026-07-20, not checkout-verified. Compounded medications are not FDA approved. This article is consumer education, not medical advice. See the methodology for how figures are gated and the public evidence ledger for per-figure sources. Found an error? Use the corrections route.