Compounded GLP-1 Vial Mix-Ups: Why Units and Milliliters Cause Overdoses

Compounded semaglutide dosing errors happen because a multi-dose vial has no built-in limit: the person drawing it up has to translate a prescription written in milligrams into a volume on a syringe marked in “units” or milliliters, and a slip of one decimal place gives ten times the intended amount. The FDA has received reports of people taking 5 to 20 times their intended dose this way, some of whom were hospitalized. The symptoms are mostly severe stomach effects, and because semaglutide stays in the body for weeks, they can last.

This page explains how the mix-up happens and which symptoms need urgent care. It is not dosing guidance; the only doses quoted below are the FDA’s own examples of errors.

What the FDA reported

In a 2024 alert on compounded semaglutide dosing errors (content current as of July 26, 2024), the FDA said it had received adverse event reports in which:

• patients measured and self-administered the wrong dose, often from multiple-dose vials, sometimes taking 5 to 20 times more than intended;
• health care professionals miscalculated doses when converting milligrams to units or milliliters, producing overdoses of 5 to 10 times the intended dose;
• some patients were given syringes much larger than the volume they needed to draw.

The FDA described three specific cases. In its words, a patient intended to take 5 units (0.05 mL) with a U-100 insulin syringe but drew up 50 units. One provider intended 0.25 mg (5 units) but prescribed 25 units, a five-fold overdose. Another intended 2 units but prescribed 20 units, a ten-fold overdose, for three patients.

The FDA’s page on unapproved GLP-1 drugs repeats that it has received “multiple reports of adverse events, some requiring hospitalization, that may be related to dosing errors.”

Units, milliliters and milligrams: why they get confused

Three different measurements are in play, and only one of them is the actual amount of drug.

On a U-100 insulin syringe, the FDA example above shows that 5 units corresponds to 0.05 mL. How many milligrams of semaglutide that volume contains depends entirely on the concentration of the particular vial, and the FDA told prescribers to be vigilant because concentrations vary between compounded products. A dose that was correct in “units” from one pharmacy can be wrong from another.

Why brand pens rarely cause this

Wegovy and Ozempic come in prefilled pens that deliver fixed, labeled amounts. The pen does the conversion for you. Pens can still be misused: a 2024 study of 152 GLP-1 exposures reported to the Alabama poison center found that 91% were therapeutic errors and 21% were related to errors on first use of a pen. But a pen cannot deliver a whole vial’s worth at once. A multi-dose vial can.

A 2023 case series from a regional poison control center made the same point. Of three patients who had obtained compounded semaglutide from compounding pharmacies and an aesthetic spa, two had self-administered ten-fold errors. One had received a vial and syringes with no pharmacist counseling, and one was dosing in milliliters and units rather than milligrams. The authors noted that vials lack the safety features of manufactured pens.

How big is the problem?

No source gives a true rate, because nobody knows how many compounded doses were used. The signals that exist all point the same way.

• A 2026 analysis of 112,532 FDA adverse event reports from 2015 to 2024 found that 63% of reports naming a GLP-1 drug as the main suspect involved administration-related reactions, compared with 39% for injectable insulin. Reports of dosing issues and administration errors for GLP-1 drugs rose from late 2022, during the national shortage when compounded vials were widespread. The authors caution that rising reports may reflect rising use.
• As of May 31, 2026, the FDA had received 990 adverse event reports associated with compounded semaglutide. It notes that state-licensed 503A pharmacies are not required to report, so the true number is likely higher.
• In a 2026 study in Public Health Reports, 20 untrained adults given gray-market semaglutide vials and supplies, and using only online information, completed on average 15 of 34 steps correctly, with errors that could lead to wrong doses.

Overdose symptoms and when to get urgent care

The FDA listed the effects reported after compounded semaglutide dosing errors: nausea, vomiting, abdominal pain, fainting, headache, migraine, dehydration, acute pancreatitis and gallstones. The Overdosage section of the Wegovy prescribing information adds that overdoses of GLP-1 drugs have caused severe nausea, severe vomiting and severe hypoglycemia, and that “a prolonged period of observation and treatment for these symptoms may be necessary.”

The label’s symptom lists for these complications are in its Medication Guide, which the table below draws on. The reason symptoms last is the drug’s half-life. The label gives semaglutide an elimination half-life of about one week, so an overdose is not cleared overnight. Symptoms may build and persist for days.

In the 2023 case series, the patient who was evaluated at a health care facility improved with an anti-nausea drug and intravenous fluids. In the Alabama poison center data, 67% of all GLP-1 exposures reported no symptoms, though not all were followed to a final outcome. Most overdoses are not life-threatening, but the serious complications above are why calling for advice is worthwhile even when you feel fine.

Checks that reduce the risk of a mix-up

The FDA’s recommendations are aimed at compounders, prescribers and patients together. Compounders and prescribers should supply a syringe of the appropriate size for the intended dose and counsel the patient on measuring it. Prescribers should check concentrations and call the compounder if unsure. Patients should talk with their prescriber or the compounder about how to measure the intended dose.

• Confirm the prescription states the dose in milligrams and the volume to draw, for this specific vial.
• Check that the vial label shows the concentration, and that it matches what the prescriber used.
• If the syringe you were given is far larger than the volume you need, ask why before using it.
• If you switch pharmacies or get a new vial, re-check; a different concentration means a different volume.
• If anything is unclear, ask the pharmacist who dispensed it to show you, before the first dose.

Products bought online without a prescription add a further problem: the stated concentration may itself be wrong. The quality testing of those products is covered in compounded semaglutide vs Ozempic, and non-sterile injection risks in peptide injection infections. For side effects at normal doses, see our GLP-1 side effects guide, and to check whether a pen is genuine, see counterfeit Ozempic signs.

If you have had a dosing error, the FDA asks patients and clinicians to report it to FDA MedWatch, which accepts medication-error reports as well as side effects.

Sources

• FDA: FDA alerts health care providers, compounders and patients of dosing errors associated with compounded injectable semaglutide products
• FDA: FDA’s Concerns with Unapproved GLP-1 Drugs Used for Weight Loss
• DailyMed: Wegovy (semaglutide) prescribing information
• Lambson JE et al. Administration errors of compounded semaglutide reported to a poison control center: case series. J Am Pharm Assoc, 2023
• Marshall S et al. GLP-1 Receptor Agonist Exposures Are Increasingly Common and Generally Associated with Mild Symptoms: A Single Poison Center Experience. J Med Toxicol, 2024
• Mattingly TJ et al. Adverse events administering glucagon-like peptide-1 receptor agonists: a cross-sectional study. Health Aff Sch, 2026
• Sedensky A et al. Ability of Adults to Correctly Use Grey Market Peptide Semaglutide GLP-1 Receptor Agonists Acquired Without a Prescription. Public Health Rep, 2026
• FDA: MedWatch Safety Information and Adverse Event Reporting Program