Compounded tirzepatide: why units and mg are not the same thing

Why this confusion exists

With a branded pen (Mounjaro or Zepbound), the dose is decided for you: you dial or select a fixed dose and the pen delivers exactly that many milligrams. There is nothing to calculate.

Compounded tirzepatide is different. It typically arrives as a vial of liquid, and you draw a dose with an insulin syringe. Those syringes are marked in units, because they were designed for insulin. And that is where the trouble starts.

Units measure liquid, not drug

A "unit" on an insulin syringe is a volume marking: on a standard U-100 syringe, 100 units is 1 millilitre. It tells you how much liquid you pulled up. It tells you nothing about how much tirzepatide is in that liquid.

How much drug you actually drew depends on the concentration of your specific vial, which is set by the compounding pharmacy and varies between pharmacies and between batches. Two vials that look identical can contain very different amounts of drug per millilitre. So the same 40 units can be a modest dose from one vial and a large overdose from another.

Why there is no universal conversion

This is the core point, and it is why you should be sceptical of any chart, forum post, or calculator that answers "how many mg is X units" with a single number. That answer only exists if you know the concentration of the exact vial in your hand. Without it, the question is unanswerable, and a confidently wrong answer is how people get hurt.

Unit-versus-milligram mix-ups with compounded GLP-1s have been reported as a cause of accidental overdose, sometimes involving many times the intended dose, with severe nausea, vomiting, and hospitalisation. The FDA has flagged dosing errors as a specific risk with compounded semaglutide and tirzepatide products.

What to do instead

• Get the concentration in writing. Your vial label and paperwork should state the concentration (drug per millilitre). If it does not, call the pharmacy and ask.
• Get your dose in both units and mg. Ask the prescriber or pharmacy to tell you exactly how many units to draw for your prescribed dose from that vial, and write it down.
• Re-check after every refill. A new vial can have a different concentration, which means your unit count changes even if your milligram dose has not.
• Never use someone else's numbers. A dose that worked for someone online refers to their vial, not yours.
• Never take a "research peptide" as a dosing reference. Unregulated powder has no verified concentration at all; see our peptide post.

The wider compounded picture

Compounded versions exist largely because of cost and shortages, and they are not the same as the FDA-approved product: the concentration, formulation, and quality control vary by pharmacy, and they are not FDA-reviewed for safety or effectiveness the way the branded drug is. That is a real trade-off to discuss with a prescriber rather than a technicality. Our compounded GLP-1 post covers it in full, and our cost post covers the legitimate ways to reduce price.

If you think you took the wrong amount

Contact your prescriber or a poison control centre promptly rather than waiting to see how it goes. In the US, Poison Control is 1-800-222-1222. Seek emergency care for persistent vomiting, inability to keep fluids down, severe abdominal pain, or signs of dehydration.