Compounded semaglutide units to mg: how the converter works

A brand semaglutide pen hides the arithmetic. A compounded vial does not. The pharmacy sends a vial with a concentration on the label, a box of insulin syringes, and a prescription in milligrams. Turning one into the other is where the errors happen, and FDA has said so: its page on unapproved GLP-1 drugs describes adverse event reports in which patients measured and self-administered incorrect doses of compounded semaglutide, some needing hospital care, and it counted 990 adverse event reports for compounded semaglutide as of May 31, 2026.

Compounded semaglutide is not FDA approved and is not interchangeable with Ozempic or Wegovy. This converter exists because people are prescribed it anyway, and the arithmetic should be right.

What "units" are

An insulin syringe is marked in units on a U-100 scale: 100 units fill 1 mL, so 1 unit is 0.01 mL. The mark measures volume. It says nothing about how much semaglutide is in that volume until you know the concentration. A prescription written in units is only meaningful together with the vial it was written for.

The three numbers

1. Concentration. Read it off the vial. Pharmacies write it two ways: a total ("5 mg in 2 mL") or a rate ("2.5 mg/mL"). The converter accepts both and shows the working concentration it derived.
2. Dose. The number your prescriber wrote, in mg, mcg, units or mL. Nothing is pre-filled.
3. Syringe. The tool shows the nearest mark on each of the three common U-100 sizes and the error each introduces.

From those it gives the volume in mL, the units to draw, the mg that the nearest mark actually delivers, and how many full doses the vial holds.

Where the errors come from

Units read as milligrams. A prescription for "0.25 mg" read as "25 units" is right at 1 mg/mL and wrong at every other concentration. At 2.5 mg/mL, 25 units is 0.625 mg, two and a half times the dose.

Carrying a concentration over. Compounded concentrations differ between pharmacies and between fills from the same pharmacy. A patient who drew 20 units from a 2.5 mg/mL vial and keeps drawing 20 units from a 5 mg/mL vial has doubled the dose. Recompute for every vial.

Rounding on the wrong syringe. Small semaglutide doses are small volumes. 0.25 mg at 2.5 mg/mL is 10 units; at 5 mg/mL it is 5 units, which on a 1 mL syringe with 2-unit marks sits between marks. The converter flags any rounding error of 5% or more in red so the syringe choice is deliberate.

Salt forms. FDA has also flagged compounded products made from semaglutide sodium or semaglutide acetate, salt forms for which it says it has no information on whether they share the properties of the approved active ingredient. A concentration on such a label does not tell you what you are measuring. That is a pharmacy question, not an arithmetic one; the compounding guide covers what to ask.

Checking with the pharmacist

The converter's output is a number to confirm, not a number to trust on its own. Read the units back to the dispensing pharmacist with the concentration and the dose, and ask them to confirm all three. If the result surprises you, stop. Draw nothing up until it is resolved.

For a more general converter that handles any drug and any container, including reconstituted vials, the dose unit converter on the FormBlends Calculators site does the same arithmetic without the semaglutide framing. FormBlends explains how its own compounded semaglutide is dispensed and labelled.