If you have ever held a compounded semaglutide or tirzepatide vial and stared at a label that says "5mg/mL" or "10mg/mL" without knowing what to do with that number, you are not alone. It is one of the most common points of confusion in the GLP-1 community — and one of the most consequential, because that number directly determines how many units you draw into your syringe.
This guide explains concentration in plain language, shows you exactly how to use it, and walks through common examples so you can dose confidently every time.
What mg/mL Actually Means
mg/mL stands for milligrams per milliliter. It tells you how much active medication is dissolved in each milliliter of liquid in your vial.
- A 5mg/mL vial contains 5 milligrams of semaglutide (or tirzepatide) in every 1 mL of liquid.
- A 10mg/mL vial contains 10 milligrams per mL — it is twice as concentrated.
- A 2.5mg/mL vial is more dilute — you need to draw more liquid to get the same dose.
The medication is the same regardless of concentration. What changes is how many units you need to draw to get your prescribed dose. This is the critical point that trips people up when switching pharmacies.
The Formula: How to Calculate Your Units
GLP-1 injections use U-100 insulin syringes, which come in 30, 50, and 100 unit sizes. All measure in the same units — 100 units equals 1 mL — so the math is the same regardless of syringe size. The formula to convert your prescribed dose (in mg) to the number of units you draw is:
Worked Examples
| Drug | Dose | Concentration | Units to Draw | Volume |
|---|---|---|---|---|
| Sema | 0.25 mg | 5 mg/mL | 5 units | 0.05 mL |
| Sema | 1 mg | 5 mg/mL | 20 units | 0.20 mL |
| Sema | 1 mg | 10 mg/mL | 10 units | 0.10 mL |
| Sema | 1 mg | 2.5 mg/mL | 40 units | 0.40 mL |
| Tirz | 2.5 mg | 10 mg/mL | 25 units | 0.25 mL |
| Tirz | 5 mg | 10 mg/mL | 50 units | 0.50 mL |
| Tirz | 5 mg | 16.6 mg/mL | 30 units | 0.30 mL |
Notice that a 1mg dose of semaglutide requires 20 units from a 5mg/mL vial, but only 10 units from a 10mg/mL vial. The dose — and the clinical effect — is identical. The syringe reading is different because the liquid is more concentrated.
Why Compounding Pharmacies Use Different Concentrations
There is no single standard concentration for compounded GLP-1 medications. Each pharmacy designs its formulation around its vial sizes, titration schedules, and patient protocols. Here is why you see so much variation:
- Vial size: A pharmacy using 1mL single-dose vials needs a higher concentration to fit a full month supply in one vial. One using 4mL multi-dose vials can use a lower concentration.
- Titration protocol: Some pharmacies ship one vial per dose level. Others ship multi-dose vials where you draw progressively more volume as your dose increases.
- Formulation additives: Many pharmacies add glycine, B12, or niacinamide. The overall concentration of all ingredients affects the ratio used for the active drug.
Common semaglutide concentrations from major compounding pharmacies: 2.5, 4, 5, and 10 mg/mL. Common tirzepatide concentrations: 5, 8, 10, 15, 16.6, and 17 mg/mL.
How to Read Your Vial Label
Every compounded vial should have a label printed by the pharmacy. Here is what to look for:
- Drug name: Semaglutide, Tirzepatide, or a compound name (e.g., Semaglutide/Glycine/B12)
- Concentration: Expressed as mg/mL — this is the number you need for your calculation
- Total volume: How many mL are in the vial (e.g., 1mL, 2mL, 4mL)
- Total mg: Often listed as well (e.g., "10mg/2mL" means 5mg/mL)
- Beyond-use date (BUD): The date after which the vial should not be used
If your label shows "10mg/2mL" but does not explicitly say "5mg/mL," divide the total mg by total mL to get the concentration: 10 ÷ 2 = 5mg/mL.
What Happens When You Switch Pharmacies
Pharmacy switches are one of the most common reasons people inject the wrong dose. If you were at Empower on a 10mg/mL tirzepatide vial and switch to a pharmacy using 5mg/mL, and you draw the same number of units — you will get half your intended dose. The opposite error results in a double dose.
The rule is simple: whenever you get a new vial — whether it is a refill, a pharmacy switch, or a different product tier — recalculate your units from scratch using the concentration on the new label. Do not assume it matches your previous vial.
For a step-by-step guide specifically on pharmacy switches, see: Switching GLP-1 Pharmacies Mid-Titration →
Calculate Your Exact Units in Seconds
Enter your dose and concentration into the free DoseCompa calculator. Select your pharmacy for automatic concentration lookup, or enter your vial's mg/mL manually.
Open the Calculator →Frequently Asked Questions
Sources
- Wilding JPH, et al. Once-Weekly Semaglutide in Adults with Overweight or Obesity. N Engl J Med. 2021;384(11):989-1002.
- Jastreboff AM, et al. Tirzepatide Once Weekly for the Treatment of Obesity. N Engl J Med. 2022;387(3):205-216.
- FDA. Human Drug Compounding: Questions and Answers. Updated 2024.
- USP <797> Pharmaceutical Compounding - Sterile Preparations. United States Pharmacopeia.