Tirzepatide & Semaglutide: Converting mg Doses to Syringe Units

If you've been prescribed compounded tirzepatide or semaglutide in a multi-dose vial, your pharmacy gave you a milligram dose, but the syringe you're drawing with is marked in units, not milligrams. Converting correctly means doing one extra step first — and skipping that step is the single most common way people accidentally over- or under-dose themselves. This is a companion to the FDA-labeled weekly dose-escalation schedule, which covers how the mg dose changes over months; this page covers the syringe math for whatever dose you're on right now.

The one number you need before you convert anything

A standard U-100 insulin syringe is calibrated so that 100 units equals 1 mL of liquid, regardless of what's dissolved in that liquid. That part is fixed. What is not fixed — and this is the part that trips people up — is how much drug is packed into each mL of a specific compounded vial. Concentration (expressed as mg/mL) is set by the compounding pharmacy that made your specific vial, and it varies meaningfully from pharmacy to pharmacy and even between batches at the same pharmacy. There is no single "standard" tirzepatide or semaglutide concentration the way there is for a brand-name pen.

That means a units chart you find online, screenshot from someone else's prescription, or reuse from a previous vial is not safe to apply to your current vial unless the concentration is identical. The only concentration that matters for your calculation is the one printed on your own vial's label or provided in your pharmacy's dosing instructions. If you don't have that number, call the pharmacy and get it before you draw a dose — do not estimate it.

The conversion formula

Once you know your vial's concentration, the math is one calculation:

**Units to draw = (your mg dose ÷ concentration in mg/mL) × 100**

Or, broken into two steps: divide your prescribed mg dose by the mg/mL concentration to get the volume in mL, then multiply that volume by 100 to get units (since a U-100 syringe reads 100 units per mL).

Worked examples

Say a compounded tirzepatide vial is labeled 10 mg/mL, and your prescribed dose is 2.5 mg (the standard tirzepatide starting dose¹). Divide 2.5 by 10 to get 0.25 mL, then multiply by 100 to get **25 units**. If instead your pharmacy's vial is labeled 5 mg/mL — a different concentration for the exact same 2.5 mg dose — the math changes: 2.5 ÷ 5 = 0.5 mL, × 100 = **50 units**. Same drug, same prescribed dose, twice as many units drawn, purely because the vial concentration is different. That's the whole risk in one example.

For semaglutide, say a vial is labeled 2 mg/mL and your dose is 1 mg (a standard maintenance step on the labeled titration schedule²). 1 ÷ 2 = 0.5 mL, × 100 = **50 units**. If that same vial were labeled 5 mg/mL instead, 1 ÷ 5 = 0.2 mL, × 100 = **20 units** — again, a real difference driven entirely by concentration, not dose.

Reference point: the FDA-labeled dose tiers

These are the milligram amounts you'd be converting into units, not the syringe math itself. For tirzepatide, the FDA-labeled dose tiers are 2.5, 5, 7.5, 10, 12.5, and 15 mg once weekly, stepping up in 2.5 mg increments per the Zepbound prescribing information¹. For semaglutide, the FDA-labeled dose tiers are 0.25, 0.5, 1, 1.7, and 2.4 mg once weekly per the Wegovy prescribing information². See the full titration schedule for how long you stay at each tier before stepping up. A compounded prescription may specify a dose within or adjacent to these tiers — the tiers above are reference points for what the brand-name, FDA-approved schedule uses, not a guarantee your compounded prescription matches them exactly.

Why this is a safety issue, not just a math inconvenience

Drawing the wrong number of units because you assumed a concentration rather than confirming it is functionally the same error as a pharmacy dispensing error — except it happens at home, without a pharmacist double-checking the calculation. Doubling your dose by using the wrong concentration is not a rounding error; it can meaningfully increase GI side effects and, at the extreme, push you toward doses well above anything tested in the trials that established these drugs' safety data. Under-dosing by the same kind of mistake just means paying for medication you're not actually getting the intended effect from. Before every new vial — not just the first one — confirm the mg/mL concentration on that specific vial's label, recalculate your units even if the mg dose hasn't changed, and mark the syringe volume down somewhere you'll see it at injection time. If any of this math doesn't add up cleanly, or your calculated units land far outside what you drew from your last vial, call the pharmacy before injecting rather than guessing. For the mechanics of the injection itself once you've drawn the correct volume, see how to actually inject semaglutide. And if you're weighing a compounded prescription in the first place, is compounded semaglutide legit and safe and the real monthly cost of compounded GLP-1s cover the two questions that usually come before this one. Not medical advice — always confirm your specific vial's concentration and your dose with your prescribing clinician or pharmacist; see our medical disclaimer.