Key takeaways
- Compounded GLP-1s come in multi-dose vials at concentrations that vary by pharmacy; the dose in milligrams is drawn as a volume using an insulin syringe marked in units, and the conversion is where errors occur.
- The FDA reported in 2024 that patients had taken 5 to 20 times their intended dose, most often by confusing units with milligrams or by using instructions written for a different concentration.
- Volume in mL equals dose in mg divided by concentration in mg/mL; units on a U-100 syringe equal mL times 100. Every dose must be recomputed if the vial concentration changes.
- Overdose produces severe nausea, vomiting, dehydration and sometimes hypoglycemia, occasionally requiring hospital care; there is no antidote, and management is supportive.
- The FDA-approved pens and single-dose vials eliminate the arithmetic, which is one of the concrete safety advantages of brand-name product.
The reports
In July 2024 the FDA issued an alert about dosing errors with compounded injectable semaglutide, later extended to tirzepatide, after receiving adverse-event reports of patients who had taken five to twenty times the intended dose. The patients had drawn the wrong volume from a multi-dose vial. Some had confused milligrams with units on an insulin syringe; some had followed instructions written for a concentration other than the one dispensed; some had been given a vial at a different concentration on refill and kept drawing the old volume. Poison-control centres reported a sharp rise in semaglutide exposure calls over the same period. The symptoms were what an overdose of a long-acting GLP-1 agonist would produce: severe nausea, vomiting, abdominal pain, dehydration and in some cases hypoglycemia, lasting days, with several patients hospitalized.
None of this happens with the FDA-approved products. Zepbound and Wegovy pens deliver a fixed dose; Zepbound single-dose vials contain one dose. The arithmetic is the compounded product's problem.
The arithmetic
A prescription says a dose in milligrams: 2.5 mg of tirzepatide, say, or 0.25 mg of semaglutide. A compounded vial says a concentration in milligrams per millilitre, and sometimes also a total content in milligrams. An insulin syringe says units, where on the standard U-100 syringe 100 units is 1 millilitre.
Volume in millilitres = dose in milligrams ÷ concentration in milligrams per millilitre. Units = millilitres × 100.
For 2.5 mg of tirzepatide from a 10 mg/mL vial: 2.5 ÷ 10 = 0.25 mL = 25 units. From a 5 mg/mL vial: 0.5 mL = 50 units. From a 20 mg/mL vial: 0.125 mL = 12.5 units. For 0.25 mg of semaglutide from a 2.5 mg/mL vial: 0.1 mL = 10 units. From a 1 mg/mL vial: 25 units. From a 5 mg/mL vial: 5 units.
Every one of those is right for its vial and wrong for the others. Tirzepatide concentrations in circulation include roughly 5, 8, 10, 12, 15, 17 and 20 mg/mL; semaglutide 1, 2, 2.5, 5 and 10 mg/mL. The tirzepatide dose calculator and semaglutide dose calculator do the division for any concentration.
The specific ways it goes wrong
Units read as milligrams. A patient reads "2.5" on the prescription and draws to the 25 line, or to the 2.5 line, without knowing what concentration makes either correct. On a 5 mg/mL vial, "25 units" is 1.25 mg, a half-dose; on a 20 mg/mL vial it is 5 mg, a double dose.
Instructions for the wrong concentration. Telehealth providers send dosing sheets; pharmacies dispense vials. When the sheet was prepared for 10 mg/mL and the pharmacy shipped 17 mg/mL, following the sheet gives 1.7 times the dose at every step.
The refill at a new concentration. A patient stable on 5 mg at 50 units from a 10 mg/mL vial receives a 20 mg/mL refill. The label changed; the habit did not. Fifty units is now 10 mg.
Total content read as concentration. A vial labelled "tirzepatide 60 mg/3 mL" is 20 mg/mL; a patient who reads "60" as the concentration under-doses by a factor of three, and one who reads the "3" as the dose volume takes the whole vial.
Copied numbers. Forums are full of "I take 25 units." That number is meaningless without the concentration behind it, and it is copied constantly.
Decimal-point errors. Semaglutide doses are small: 0.25 mg versus 2.5 mg is one misplaced point and a ten-fold overdose. The FDA's reports included exactly that.
What an overdose looks like and what to do
Because both drugs have half-lives of five to seven days, an overdose is not a bad afternoon; it is a bad week. Nausea and vomiting can be severe enough to prevent fluid intake, producing dehydration that has caused acute kidney injury in reported cases. Hypoglycemia can occur, particularly in patients on insulin or sulfonylureas, and has been reported even without them at high doses. Abdominal pain should prompt evaluation for pancreatitis.
There is no antidote. Management is supportive: fluids, anti-emetics, glucose monitoring, and hospital care if oral intake fails. Anyone who realizes they have taken a substantially larger dose than intended should contact poison control (1-800-222-1222 in the United States) and their prescriber, and seek emergency care for persistent vomiting, inability to keep fluids down, confusion, or symptoms of low blood sugar. Do not take the next scheduled dose until a clinician says to.
The checklist
Before every injection, in this order:
- Read the concentration on the vial itself, in mg/mL. Not the box. Not the instruction sheet.
- Confirm the prescribed dose in milligrams from the prescription or the provider's portal.
- Compute the volume: mg ÷ mg/mL = mL. Multiply by 100 for units on a U-100 syringe. Use the calculator if in doubt.
- Compare the result with the provider's instructions. If they disagree, do not inject; call the pharmacy.
- Draw the volume and read the syringe against the number you computed, not against memory.
- Whenever a new vial arrives, start again at step one, even if it looks the same.
- Keep a written log: date, vial concentration, dose, units.
Why this is a reason to weigh brand-name product
The arithmetic is avoidable. A pen delivers the dose. A single-dose vial contains the dose. The price difference between compounded and brand-name tirzepatide at maintenance doses is roughly $250 to $310 per month on our figures, which is a great deal of money, and for most patients the checklist above reduces the compounded risk to something manageable. But a patient who is not confident with the arithmetic, who has had an error, or who shares a household with someone who might use the vial, has a concrete safety reason to consider the approved product that no price comparison captures. The brand versus compounded article sets the trade-off out in full.
What providers and pharmacies should do
Dispense with instructions that state the concentration of the vial actually shipped, and reissue them on any change. Label vials in mg/mL prominently, not only in total content. Use syringes graduated in a way that matches the dose volumes prescribed. Confirm concentration with the patient at the first refill. Several providers in our database do some of these; none we have reviewed does all of them, and none discloses its error rate. Readers who have experienced a dosing error with a named provider can tell us; we record such reports with dates.
Frequently asked questions
How many units is 2.5 mg of tirzepatide?
It depends on the vial concentration. At 10 mg/mL, 2.5 mg is 0.25 mL, or 25 units on a U-100 insulin syringe. At 5 mg/mL it is 50 units; at 20 mg/mL it is 12.5 units. Read your own vial.
What happens if you inject too much tirzepatide or semaglutide?
Severe nausea, vomiting, abdominal pain and dehydration, sometimes hypoglycemia, lasting days because of the drugs' long half-lives. Serious cases have needed hospital care. There is no antidote; treatment is fluids, anti-nausea medication and monitoring. Call poison control or seek care if it happens.
Why don't compounded vials come in fixed doses?
Because compounding pharmacies prepare multi-dose vials to their own formulations, and the FDA-approved pens and single-dose vials are the products the manufacturers make. The variable concentration is a feature of compounding, and it is the root of the error.
Sources
- FDA. FDA alerts health care providers, compounders and patients of dosing errors associated with compounded injectable semaglutide products. July 26, 2024 (updated 2025).
- FDA. FDA's Concerns with Unapproved GLP-1 Drugs Used for Weight Loss. 2025.
- Institute for Safe Medication Practices. Errors with compounded semaglutide and tirzepatide. ISMP Medication Safety Alert, 2024.
- Zepbound (tirzepatide) prescribing information, Eli Lilly, 2025 (dosage and administration).
- Wegovy (semaglutide) prescribing information, Novo Nordisk, 2025 (dosage and administration).
- America's Poison Centers. Semaglutide exposure calls, 2023-2024 data.
Citations are to primary sources (peer-reviewed trials, FDA labeling and announcements, and provider pricing pages). See our sources policy and corrections log.