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Three different measures end up on the same label, prescription and syringe. Milligrams (mg) are the amount of drug. Millilitres (mL) are the amount of liquid. Units are the marks printed on an insulin syringe, and each one is 0.01 mL. Mixing up any two of them is how a dose ends up several times too large or too small.
Informational use only — not medical advice. Check every figure against your own product and talk to a licensed healthcare provider before starting or changing anything.
| Measure | What it counts | Where you see it |
|---|---|---|
| mg (milligrams) | The drug itself | The prescription or dose instruction; the total on the vial label |
| mL (millilitres) | The liquid the drug is dissolved in | The vial's fill volume; the strength as mg per mL |
| units | Marks on an insulin syringe, 0.01 mL each | The syringe barrel; some compounders' instructions |
The link between them is the vial's strength in mg/mL. Without it, a number of units says nothing about how much drug is in the syringe.
Worked example: a 0.25 mg dose from a vial at 5 mg/mL is 0.25 ÷ 5 = 0.05 mL, which is 5 units. From a weekly vial of Tirzepatide at 10 mg/mL, 2.5 mg is 0.25 mL, or 25 units.
Compounded vials come in more than one strength, and the strength can change between refills. The mg stay the same; the units you draw do not. Figures for Semaglutide at two strengths:
| Dose | mL at 5 mg/mL | Units at 5 mg/mL | Units at 2.5 mg/mL |
|---|---|---|---|
| 0.25 mg | 0.05 mL | 5 units | 10 units |
| 0.5 mg | 0.1 mL | 10 units | 20 units |
| 1 mg | 0.2 mL | 20 units | 40 units |
| 1.7 mg | 0.34 mL | 34 units | 68 units |
| 2.4 mg | 0.48 mL | 48 units | 96 units |
A dose that was 5 units on the first vial is 10 units on a vial half as strong. Drawing the old number from the new vial gives half the dose; the reverse gives double.
On 26 July 2024 the FDA alerted providers, compounders and patients to dosing errors with compounded injectable semaglutide. It received reports of patients who gave themselves five to 20 times the intended dose from multiple-dose vials, some of whom sought medical attention or needed hospital care. It said inexperience with drawing from a vial, "coupled with confusion between different units of measurement", may have contributed, and that some instructions are written in units rather than mg or mL. Its advice to patients: talk with your health care provider or compounder about how to measure and administer the intended dose.
The mcg to units chart lists common doses at common strengths side by side.
No. A unit is a volume mark: 0.01 mL on any U-100 insulin syringe. How many mg that holds depends on the vial's strength. At 5 mg/mL, 1 unit holds 0.05 mg; at 2.5 mg/mL it holds 0.025 mg.
Divide the dose in mg by the vial's concentration in mg/mL to get mL, then multiply by 100 to get units. Units = mg ÷ (mg/mL) × 100.
Multiply by 100. On a U-100 syringe, 0.1 mL is 10 units and 0.5 mL is 50 units. This holds no matter what is in the vial.
Because units measure liquid, not drug. If a new vial is half as strong, the same mark holds half the mg. Check the mg/mL printed on every new vial and recalculate before the first draw.
In July 2024 the FDA reported patients who drew five to 20 times the intended dose of compounded semaglutide from multiple-dose vials, and said confusion between milliliters, milligrams and units may have contributed. It asked compounders to supply a syringe sized for the dose and to show patients how to measure it.
Dosify Me does this math for you. Pick the compound, your vial size and dose, and the app draws the exact syringe reading, keeps your schedule and reminders, and logs every dose — privately, on your phone.
Figures computed from the Dosify Me compound database on 2026-10-08. FDA alert read on 20 October 2026.