The short answer
Your prescription is in milligrams, but the syringe measures volume. On a standard U-100 insulin syringe, 100 units equal 1 mL, so 10 units is 0.1 mL. How much drug that holds depends entirely on your vial’s concentration: at 2.5 mg/mL, 10 units is 0.25 mg; at 5 mg/mL, the same 10 units is 0.5 mg. Read the mg/mL on your pharmacy label and have the pharmacist confirm your dose in mg, mL and units before you draw it.
At GLP-1 SOS we often hear from people who switched from a branded pen to a compounded vial and were handed a number of units with no explanation. The pen did the math for you. A vial does not. This page shows the math so you can check it, and the one person who should confirm it: your pharmacist.
At a glance
Three numbers, one dose
1
Milligrams are the dose
Your prescriber chooses a dose in mg, like 0.25 mg of semaglutide or 2.5 mg of tirzepatide.
2
Units are volume
On a U-100 syringe, each unit is 0.01 mL. Units say how far to pull the plunger, not how much drug.
3
Concentration links them
The mg/mL on your vial turns volume into dose. Change the vial, and the units for the same dose can change.
Why the same number of units can be a different dose
Approved pens are dosed in milligrams and come in fixed strengths. Compounded semaglutide and tirzepatide are not FDA-approved, and the FDA notes that their concentrations vary between pharmacies, a single pharmacy may sell several, and instructions are often written in “units”, a volume that means a different dose at each concentration. Some vials must also be mixed first, which sets the concentration; our page on storing and mixing compounded GLP-1s covers that step.
The approved labels show the same principle. Every dose from a Zepbound multi-dose vial is 0.6 mL, yet that 0.6 mL holds 2.5 mg in the 4.17 mg/mL vial and 15 mg in the 25 mg/mL vial. Volume alone tells you nothing about the dose.
Lab note
Units tell you how far to pull the plunger. Only the concentration tells you how much medicine is in it.
The math is two steps. First, milliliters = your dose in mg divided by the concentration in mg/mL. Second, units on a U-100 syringe = milliliters x 100. For 0.25 mg from a 2.5 mg/mL vial: 0.25 / 2.5 = 0.1 mL, which is 10 units. If the label gives only a total, such as 10 mg in 4 mL, divide to get mg/mL (2.5) first. These are examples to show the method, not your dose.
In a 2024 alert, the FDA described dosing errors with compounded semaglutide, some needing hospital care. Most reports involved people drawing up too much, 5 to 20 times the intended dose: told to take 5 units (0.05 mL) on a 1 mL syringe, they took 50. Prescribers made errors too, writing 25 units when 0.25 mg was 5 units for that product. Reported effects included severe nausea and vomiting, fainting, dehydration, pancreatitis and gallstones.
From the FDA and the labels
The numbers to know
100 units = 1 mL
on a standard U-100 insulin syringe, so each small line on a 1 mL syringe is 1 unit, or 0.01 mL (MedlinePlus)
5 to 20 times
the intended dose that people drew up in the dosing errors the FDA reviewed with compounded semaglutide vials (FDA, 2024)
0.6 mL
the volume of every dose from a Zepbound multi-dose vial, holding anywhere from 2.5 mg to 15 mg depending on the vial (Zepbound label)
Sources: FDA alert on dosing errors with compounded semaglutide, July 2024; MedlinePlus, Giving an insulin injection; Zepbound Prescribing Information, August 2026.
How to get the dose right every time
- Find the concentration on your vial label. Look for mg/mL. If the label shows a total, like 10 mg in 4 mL, divide to get it. If you see only units, call the pharmacy before your first dose.
- Write your dose three ways. On the box: dose in mg, the volume in mL, and the units on your syringe. Ask the pharmacist to confirm: for this vial, my dose of __ mg is __ mL, or __ units.
- Use the syringe the pharmacy matched to your dose. The FDA saw errors with syringes much larger than the dose and asks pharmacies to supply the right size. Ask them to show you the exact line for your dose.
- Recheck whenever anything changes. New vial, new pharmacy, new concentration or new dose: the units change even when the mg does not. Never carry an old unit number over.
- Do not recalculate from a forum or an online calculator. In the FDA reports, a patient who searched online took five times the dose, and a health care provider who recalculated their own dose took ten times. Your pharmacy knows its product.
- Step up only as prescribed, and support your gut on dose-step weeks. The FDA has flagged faster-than-label dose increases with compounded products. On a step, expect a few rougher days, as in dose increase side effects. GLP-1 SOS Daily Gut Relief, 2 capsules with dinner, supports regular bowel movements and digestive comfort while you adjust.*
*GLP-1 SOS Daily Gut Relief supports regular bowel movements and digestive comfort. These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure or prevent any disease.
Before you inject
Go ahead, or stop and call first
Fine to go ahead
- The label shows the concentration in mg/mL
- Your mg, mL and units all agree
- The syringe size fits the dose
- The pharmacist confirmed it for this vial
Stop and call the pharmacy
- Instructions give units only, with no mg or mg/mL
- Your numbers do not agree
- A new vial or pharmacy with a different concentration
- You cannot read the syringe markings clearly
A two-minute call to the pharmacy costs less than a week of vomiting.
When to call your doctor
Call your prescriber the same day, or Poison Help at 1-800-222-1222, if:
- You think you injected more than your dose, even if you feel fine now
- You have severe nausea or vomiting and cannot keep liquids down
- You have severe belly pain that spreads to your back, a possible sign of pancreatitis
- You feel faint, very dizzy or pass out
- You take insulin or a sulfonylurea and feel shaky, sweaty or confused
- You are barely urinating or your urine is very dark
Semaglutide stays in the body for about a week and tirzepatide about five days, so an overdose can take days to settle. Call early.
From the people behind the Lab
You do not have to figure this out alone.
GLP-1 SOS is the daily companion made for life on a GLP-1. Plain answers like this one, a support team that replies to real questions from real people, and daily formulas like GLP-1 SOS Daily Gut Relief, designed around the side effects that come with the medication. The GLP-1 Lab is our free education arm: no login, no paywall, no hype. Side effects are part of the road. Suffering through them is not.
Questions people also ask
How many units is 0.25 mg of compounded semaglutide?
It depends on your vial’s concentration, so there is no single answer. At 2.5 mg/mL, 0.25 mg is 0.1 mL, or 10 units on a U-100 syringe. At 5 mg/mL, it is 0.05 mL, or 5 units. Read the mg/mL on your label, do the math, and have your pharmacist confirm the number for your exact vial.
How many units is 2.5 mg of compounded tirzepatide?
Again, it depends on the concentration. At 10 mg/mL, 2.5 mg is 0.25 mL, or 25 units on a U-100 syringe. At 5 mg/mL, it is 0.5 mL, or 50 units. That is why a unit number copied from someone else, or from your last vial, can be wrong. Confirm with the pharmacy that filled your vial.
Is 10 units the same as 1 mg?
Not as a rule. Units on an insulin syringe measure volume: 10 units is 0.1 mL. The milligrams in that 0.1 mL depend on the vial: 0.25 mg at 2.5 mg/mL, 0.5 mg at 5 mg/mL, 1 mg at 10 mg/mL. Mixing up units and milligrams is one of the errors behind the overdoses the FDA reported.
My new vial has a different concentration. Do I keep the same units?
No, not without recalculating. Your dose in milligrams stays the same, but the volume changes with the concentration, so the units change too. A stronger vial needs fewer units for the same dose. Ask the pharmacist to confirm the new number in mg, mL and units, and write it on the new box.
What should I do if I injected too much compounded semaglutide?
Call Poison Help at 1-800-222-1222 or your prescriber now, even if you feel fine; symptoms can build over days. The labels mention severe nausea, vomiting and low blood sugar after overdoses. Sip fluids, ask before taking your next dose, and see side effects on compounded GLP-1s.
Sources (5)
- FDA alerts health care providers, compounders and patients of dosing errors associated with compounded injectable semaglutide products. U.S. Food and Drug Administration, July 26, 2024.
- FDA’s Concerns with Unapproved GLP-1 Drugs Used for Weight Loss. U.S. Food and Drug Administration, current as of October 1, 2026.
- Giving an insulin injection. MedlinePlus Medical Encyclopedia, U.S. National Library of Medicine, reviewed July 2024.
- Zepbound (tirzepatide) injection, Prescribing Information. Eli Lilly, revised August 2026.
- Wegovy (semaglutide) injection and tablets, Prescribing Information. Novo Nordisk / FDA, revised June 2026.
Reviewed by the GLP-1 SOS editorial team · Education, not medical advice · Talk to your prescriber about your dose and your symptoms.
