Semaglutide Units: How the mg-to-Units Math Actually Works

Somebody told you to draw “20 units.” Your prescription says 0.5 mg. Those are not the same kind of number, and the thing that connects them is printed on your vial in small type.

This is not a niche confusion. On July 26, 2024, FDA issued a formal alert after receiving reports of adverse events, some requiring hospitalization, from overdoses of compounded semaglutide. Most of the reports described patients drawing up more than the prescribed dose from a multiple-dose vial. In those cases, patients administered five to twenty times the intended amount [1][2].

FDA named the cause in plain language: “Unfamiliarity with withdrawing medication from a vial into a syringe and coupled with confusion between different units of measurement (e.g., milliliters, milligrams and ‘units’) may have contributed to dosing errors” [1].

This article explains the arithmetic so you can check a number someone gave you. It is not a dose recommendation and it does not tell you what you should be taking. For FDA-approved products it summarizes what the approved Instructions for Use say; for compounded product there is no FDA-approved Instructions for Use at all, which is precisely why the arithmetic matters. In either case, follow your own prescription and the instructions your pharmacy supplied with your specific vial, and confirm any figure you calculate with your prescriber or pharmacist before you draw it.

Why is my dose in milligrams but my syringe in units?

Because they were designed for different things.

Your prescription is written in milligrams, the same as every FDA-approved semaglutide product. Brand-name Ozempic, Wegovy and Rybelsus are all dosed in milligrams, at fixed concentrations, in devices that either dial a milligram dose or contain one fixed dose [4].

Compounded semaglutide is different. It usually arrives as a liquid in a multi-dose vial, and it is drawn up with an insulin syringe. Insulin syringes are marked in units, because insulin genuinely is dosed in units of biological activity.

Semaglutide is not. Neither is tirzepatide. Neither has any “unit” in that sense. So when an instruction says “draw to 20 units,” it is describing a volume of liquid and nothing else.

FDA’s alert and the trade coverage of it make the same point: FDA-approved semaglutide products are dosed in milligrams at standard concentrations, while instructions for a compounded drug “may instruct users to administer semaglutide injections in ‘units,’ the volume of which may vary depending on the concentration” [5].

What is a “unit” on an insulin syringe?

A U-100 insulin syringe is calibrated so that 100 units equals 1 milliliter.

That is the whole definition. One unit is exactly 0.01 mL. It is a volume marking on a barrel. The syringe has no idea what liquid is inside it.

So:

• 10 units = 0.10 mL
• 20 units = 0.20 mL
• 50 units = 0.50 mL
• 100 units = 1.00 mL

U-100 is the standard insulin syringe sold in the United States, and it is what conversion charts assume. U-40 and U-500 syringes also exist, and they use different calibrations. If you happen to have one — leftover veterinary supplies are the usual route — the math above does not apply and the dose would be wrong.

What is the formula?

Two numbers you have, one number you need.

Milligrams to units:

units = (dose in mg ÷ concentration in mg/mL) × 100

Units back to milligrams:

mg = (units ÷ 100) × concentration in mg/mL

Units to volume, always:

mL = units ÷ 100

The concentration is the only input you cannot work out for yourself. It comes off the vial label or the pharmacy paperwork.

Can you show me the math worked out?

Here it is step by step, using the two concentrations US compounding pharmacies most commonly dispense for semaglutide [6].

Example 1. 0.25 mg from a 2.5 mg/mL vial.

• 0.25 ÷ 2.5 = 0.1 mL
• 0.1 × 100 = 10 units

Example 2. The same 0.25 mg from a 5 mg/mL vial.

• 0.25 ÷ 5 = 0.05 mL
• 0.05 × 100 = 5 units

Same prescription. Half the number on the syringe. This is the entire problem in two lines.

Example 3. 2.4 mg from a 2.5 mg/mL vial.

• 2.4 ÷ 2.5 = 0.96 mL
• 0.96 × 100 = 96 units

Example 4. Going backward. You drew 30 units from a 5 mg/mL vial. How much did you take?

• 30 ÷ 100 = 0.3 mL
• 0.3 × 5 = 1.5 mg

Is there a table I can just read?

Yes, but read the concentration column heading first, every single time.

Semaglutide, units on a U-100 syringe

Dose	at 1 mg/mL	at 2 mg/mL	at 2.5 mg/mL	at 5 mg/mL	at 10 mg/mL
0.25 mg	25 units	12.5 units	10 units	5 units	2.5 units
0.5 mg	50 units	25 units	20 units	10 units	5 units
1 mg	100 units	50 units	40 units	20 units	10 units
1.7 mg	170 units	85 units	68 units	34 units	17 units
2 mg	200 units	100 units	80 units	40 units	20 units
2.4 mg	240 units	120 units	96 units	48 units	24 units

Tirzepatide, units on a U-100 syringe

Dose	at 5 mg/mL	at 10 mg/mL	at 20 mg/mL
2.5 mg	50 units	25 units	12.5 units
5 mg	100 units	50 units	25 units
7.5 mg	150 units	75 units	37.5 units
10 mg	200 units	100 units	50 units
15 mg	300 units	150 units	75 units

Everything in both tables is computed from the formula above, not copied from a chart. You can check any cell yourself in ten seconds.

Two shortcuts, if the mental arithmetic helps: at 2.5 mg/mL, multiply milligrams by 40 to get units. At 5 mg/mL, multiply by 20. At 10 mg/mL, multiply by 10.

Notice the cells above 100 units. A 1 mL U-100 syringe cannot hold them. That is a real constraint, not a rounding issue, and it is why higher-dose patients are often moved to a more concentrated vial.

How do I find my concentration?

Look at the vial label. You will see one of three things.

Best case: total drug and total volume. Something like “Semaglutide 10 mg / 2 mL.” Divide: 10 ÷ 2 = 5 mg/mL. You now have everything you need.

Also workable: strength per mL. Something like “5 mg/mL.” Use it directly.

A problem: total milligrams only. Something like “Semaglutide 10 mg,” with no volume. This does not tell you the concentration, because the same 10 mg of drug can be dissolved in 1 mL, 2 mL or 4 mL. The concentration is in the pharmacy’s dispensing paperwork, the patient instruction sheet, or your patient portal. Call and ask. Do not estimate it.

FDA has written about exactly this. In a May 21, 2024 alert, the agency warned that compounders often display strength per mL prominently while conventional manufacturers display strength per total volume, and that clinicians reading the wrong number have administered five to fifty times the intended dose of other small-volume injectables. FDA’s recommendation is that “strength per total volume should be the primary and prominent expression on the principal display panel of the label, followed in close proximity by strength per milliliter enclosed by parentheses” [3].

That is why an FDA-approved tirzepatide multi-dose vial reads “40 mg/2.4 mL (16.7 mg/mL).” It follows the convention. A compounded vial may not.

A special case: powder. A lyophilized (freeze-dried) vial has no concentration at all until liquid is added. The concentration is created at reconstitution, by how much liquid goes in. A 5 mg powder vial plus 2 mL of bacteriostatic water is 2.5 mg/mL. The same vial plus 1 mL is 5 mg/mL — double, for every subsequent draw. If you were sent a powder vial, the reconstitution instructions that came with your specific product are the only ones that apply to it.

What actually goes wrong, according to FDA?

The July 2024 alert is unusually specific about the failure modes, and they are worth reading because they are all recognizable [1][2][7]:

Reading the syringe wrong. “Several reports showed patients were told to use U-100 insulin (1 mL) syringes to draw up small doses. Rather than filling the syringe as instructed with 5 units (0.05 mL), these patients mistakenly administered 50 units instead” [7]. That is a tenfold overdose from a single misread.

The prescriber converting wrong. In one case, a provider intended to prescribe 0.25 mg — 5 units at that concentration — and wrote 25 units. Five times the intended dose. In another, a provider prescribed 20 units instead of 2 units, and three patients received ten times their intended dose [1][7].

Nobody available to ask. FDA describes a case in which a patient could not get clarity on dosing instructions from the telemedicine provider who wrote the prescription, searched online for medical advice instead, and took five times the intended dose [1].

A syringe that was too big for the job. Some reports noted that patients received syringes significantly larger than the prescribed volume, which makes a small dose harder to measure accurately [5].

The reported harms were not trivial: gastrointestinal effects including nausea, vomiting and abdominal pain, plus fainting, headache, migraine, dehydration, acute pancreatitis and gallstones. Some patients sought medical attention or required hospitalization [1]. A separate 2023 case series reported three adverse drug events after incorrect administration of semaglutide obtained from compounding pharmacies and an aesthetic clinic [8].

FDA’s recommendation to prescribers is direct: “provide patients with the appropriate syringe size for the intended dose and counsel patients on how to measure” it. To patients: “talk with their health care provider or compounder about how to measure and administer the intended dose” [1].

What are the most common ways people get this wrong?

Based on what FDA documented and what shows up repeatedly in patient forums [9]:

Switching pharmacies and reusing the old unit number. Your dose in milligrams did not change, so you kept drawing the same number of units. But the new vial is a different concentration, so you are now taking half or double. Re-check the concentration on every new vial, including refills.

Copying a number from a stranger. “I take 20 units” is meaningless without the concentration behind it. Two people can both draw 20 units and take completely different amounts.

Using a chart for the wrong concentration. Charts circulate widely and often do not name their assumption prominently. Check the column heading against your vial before you use any row.

Counting the lines instead of the numbers. On many U-100 syringes the printed numbers are every 10 units and the small lines are every 2 units, so the mark above the 10 is 12, not 11. A patient posting in a semaglutide forum in September 2026 described doubling their dose because they read “8 units” as eight lines [9]. Look at what the barrel is actually graduated in before the first draw.

Assuming a brand pen can be used with a syringe. It cannot. The Ozempic Instructions for Use says: “Never use a syringe to withdraw OZEMPIC from your pen” [4]. The Zepbound KwikPen instructions say the same: “Do not transfer ZEPBOUND from your Pen into a syringe.”

Does any of this apply to brand-name products?

No, and that is worth saying clearly, because it is the cleanest way to avoid the whole problem.

FDA-approved semaglutide and tirzepatide are never dosed in units [1][5]:

• Ozempic pens dial a milligram dose on a dose counter. There is nothing to convert.
• Wegovy single-dose pens and prefilled syringes contain one fixed dose each. You use the whole thing.
• The Wegovy FlexTouch delivers four fixed 2.4 mg doses. No dialing arithmetic.
• Zepbound and Mounjaro single-dose vials are a fixed 0.5 mL. Multi-dose vials are a fixed 0.6 mL. The label tells you to use a 1 mL syringe and draw to that line [4].
• Rybelsus, Ozempic tablets, Wegovy tablets and Foundayo are tablets. There is no measuring at all.

If unit conversion is a source of anxiety for you, that is a reasonable thing to raise with your prescriber when you are discussing options.

What to do if you think you drew the wrong amount

This article cannot tell you how serious a particular error is, and it should not try.

If you think you have taken more than your prescribed dose, contact your healthcare provider. FDA’s alert lists the overdose effects it received reports of — nausea, vomiting, abdominal pain, fainting, headache, migraine, dehydration, acute pancreatitis and gallstones — and notes that some patients required hospitalization [1]. The Medication Guides for these drugs direct people who have taken too much to call their healthcare provider or the Poison Help line at 1-800-222-1222, or go to the nearest emergency room [4].

FDA also asks that dosing errors and adverse events with compounded semaglutide be reported to its MedWatch program [1].

The one-line version

A unit is 0.01 mL. Your dose in milligrams divided by your vial’s concentration in mg/mL, times 100, is the number of units. If you do not know the concentration, you do not have enough information to draw a dose — and that is a phone call, not a guess.

Sources

1. US Food and Drug Administration. “FDA alerts health care providers, compounders and patients of dosing errors associated with compounded injectable semaglutide products,” July 26, 2024. https://www.fda.gov/drugs/human-drug-compounding/fda-alerts-health-care-providers-compounders-and-patients-dosing-errors-associated-compounded
2. US Food and Drug Administration, MedWatch bulletin of the same alert, July 26, 2024. https://content.govdelivery.com/accounts/USFDA/bulletins/3ab6dfa
3. US Food and Drug Administration. “Differences in strength expression on product labels of compounders and conventional manufacturers may lead to dosing errors,” May 21, 2024. https://www.fda.gov/drugs/human-drug-compounding/differences-strength-expression-product-labels-compounders-and-conventional-manufacturers-may-lead
4. Novo Nordisk. OZEMPIC (semaglutide) injection — US Prescribing Information, Medication Guide and Instructions for Use, June 2026. https://www.novo-pi.com/ozempic.pdf
5. Medscape. “Compounded Semaglutide Overdoses Tied to Hospitalizations,” July 2024. https://www.medscape.com/viewarticle/compounded-semaglutide-overdoses-tied-hospitalizations-2024a1000dte
6. Weight Loss Rankings. “GLP-1 Units to mg Dosage Chart: Semaglutide & Tirzepatide,” April 7, 2026 — used only to corroborate which concentrations US compounding pharmacies commonly dispense. https://www.weightlossrankings.org/research/glp1-units-to-mg-dosage-chart
7. Renal & Urology News. “FDA: Dosing Errors, Adverse Events Reported With Compounded Semaglutide,” July 2024. https://www.renalandurologynews.com/news/fda-dosing-errors-adverse-events-reported-with-compounded-semaglutide
8. Lambson JE et al. “Administration errors of compounded semaglutide reported to a poison control center — case series,” 2023. https://pubmed.ncbi.nlm.nih.gov/37392810
9. r/Semaglutide. “I’m new to self injections with a syringe, was confused by what 8 units is,” September 6, 2026 — cited as evidence that the confusion is current, not as a medical source. https://www.reddit.com/r/Semaglutide/comments/1w908fy/im_new_to_self_injections_with_a_syringe_was/