Dosing compounded semaglutide or tirzepatide from a vial? Here is how units, mg and mL relate on a U-100 syringe, with worked examples and safety tips.
If you get your GLP-1 from a telehealth provider or compounding pharmacy, you may be drawing doses from a vial yourself. The math trips a lot of people up. Here’s how it works.
| Term | What it means |
|---|---|
| mg (milligrams) | How much medicine. Your dose is usually written this way |
| mL (milliliters) | How much liquid |
| Units | The markings on an insulin syringe. On a U-100 syringe, 1 unit = 0.01 mL |
Your vial strength, printed on the label as mg/mL, links them.
Units = (dose in mg ÷ vial strength in mg/mL) × 100
| Medicine | Vial strength | Dose | Units to draw | mL |
|---|---|---|---|---|
| Semaglutide | 2.5 mg/mL | 0.25 mg | 10 units | 0.10 mL |
| Semaglutide | 2.5 mg/mL | 1 mg | 40 units | 0.40 mL |
| Tirzepatide | 10 mg/mL | 2.5 mg | 25 units | 0.25 mL |
| Tirzepatide | 10 mg/mL | 5 mg | 50 units | 0.50 mL |
The same mg dose can be a very different number of units in different vials. If your pharmacy changes your vial strength, your units change too.
Compounded medicines are made by a pharmacy for an individual patient. They aren’t FDA-approved, which means the FDA hasn’t reviewed their safety, effectiveness or quality. Many people use them. Just know that the label-based guidance for brand medicines, like missed-dose rules, may not apply. Ask your prescriber.
Holly, 41, gets compounded tirzepatide. Her first vial was 10 mg/mL, so 2.5 mg was 25 units. Her refill arrives at a different strength. Before injecting, she checks the label, recalculates, and calls her pharmacy to confirm the new number of units. It takes five minutes and prevents a doubled dose.
Look for:
Follow your pharmacy’s instructions. Many compounded vials need refrigeration and have a use-by date once opened. There is no single answer here — compounded products are made to each pharmacy’s own formula, so unlike a branded pen there is no label you can look up. Don’t use a vial that looks cloudy or discolored unless your pharmacy says that’s expected.
It depends on your vial. At 2.5 mg/mL it’s 10 units on a U-100 syringe. Always confirm with your pharmacy.
At 10 mg/mL it’s 25 units on a U-100 syringe. Other vial strengths will be different.
Recalculate your units and confirm with the pharmacy before your next dose.
They aren’t FDA-approved, so their quality can vary. Use a licensed pharmacy and talk to your prescriber.
An insulin syringe where 100 units equals 1 mL. It’s the most common type used for compounded GLP-1s.
Use what your pharmacy supplies and explains. Different sizes have different markings.