People switch compounding pharmacies all the time — for cost, availability, a provider switch, or because their current pharmacy stopped carrying the formulation they need. It is a normal part of the GLP-1 journey. But it comes with one critical risk that is not always explained clearly: the new pharmacy almost certainly uses a different concentration, which means your unit count changes even if your dose in milligrams stays the same.

Drawing the same number of units from a new vial without recalculating is one of the most common dosing errors in the GLP-1 community. This guide walks through exactly what to do so you can make the transition safely, without losing your titration progress or injecting the wrong amount.

⚠ The Most Common Mistake You were drawing 30 units from your old pharmacy's 5mg/mL vial (= 1.5mg dose). Your new vial is 10mg/mL. If you draw 30 units from the new vial, you inject 3mg — double your intended dose. Always recalculate before your first injection from any new vial.

The Core Principle: Keep Your Dose (mg), Not Your Units

Your dose is measured in milligrams — that is what your provider prescribed and what your body responds to. The number of units you draw is just a way of measuring that dose using your syringe. When the concentration changes, the units change but the milligrams do not.

Think of it like a currency conversion. If you know you need $10 worth of something, and the exchange rate changes, you need more or fewer bills to get the same value. The value — your dose in mg — stays the same. The unit count adjusts to match the new concentration.

Step-by-Step: How to Recalculate

1

Find your current prescribed dose in mg

Check your prescription, your provider's titration schedule, or your previous pharmacy paperwork. You need the milligram number — not the unit count. If your prescription only lists units and a concentration, do the reverse math: Units ÷ 100 × old concentration = mg.

2

Read the new vial's concentration

Look at the pharmacy label on your new vial. Find the mg/mL number. If the label shows "10mg/2mL" or similar, divide: 10 ÷ 2 = 5mg/mL. If you cannot find the concentration, call the pharmacy before injecting — do not guess.

3

Apply the formula

Units = (Dose in mg ÷ Concentration in mg/mL) × 100. This gives you the number of units to draw on a standard U-100 insulin syringe.

4

Verify with a calculator

Use the DoseCompa calculator to confirm your math before drawing. Enter your dose and the new concentration. If your pharmacy is listed, select it and the concentration will auto-fill.

5

Confirm with your provider if unsure

If there is any doubt — the label is unclear, the concentration jump seems extreme, or you are on a complex protocol — message or call your provider before the first injection from the new vial.

Units = (Dose mg ÷ Concentration mg/mL) × 100
Using a U-100 insulin syringe · 100 units = 1 mL

Real Switching Scenarios

ScenarioOld UnitsNew Conc.New UnitsSame Dose?
Sema 1mg: 5→10 mg/mL20 u10 mg/mL10 uYes, if recalculated
Sema 1mg: no recalc20 u drawn10 mg/mL20 u drawnNo — double dose
Tirz 5mg: 10→5 mg/mL50 u5 mg/mL100 uYes, if recalculated
Tirz 5mg: Empower→Wells62.5 u16.6 mg/mL30 uYes, if recalculated
Ozempic 1mg pen → comp. 5mg/mLpen click5 mg/mL20 uYes, draw 20 units

Switching from a Branded Pen to a Compounded Vial

This is a specific case worth addressing directly, because it is increasingly common. Many people who started on Ozempic or Wegovy switch to compounded semaglutide for cost reasons. The transition works like this:

Example: You were on Ozempic 1mg (one pen click). Your new compounded vial is semaglutide 5mg/mL. You draw: (1 ÷ 5) × 100 = 20 units.

💡 Tiered Protocol Pharmacies Some pharmacies — notably VialsRx and Greenwich Rx — use a system where each monthly shipment is a different vial with a concentration matched to that month's prescribed dose. The unit count you draw may stay the same each month even as your dose increases, because the concentration increases proportionally. The rule still applies: check each vial's concentration and recalculate if anything changes.

What to Do if You Don't Know Your Current Dose in mg

If your previous pharmacy or provider only told you how many units to draw — without specifying the milligrams — you can reverse-calculate your dose:

Dose (mg) = (Units drawn ÷ 100) × Concentration (mg/mL)
Example: 30 units from a 5mg/mL vial = (30 ÷ 100) × 5 = 1.5mg

Once you know your dose in mg, you can calculate the units for any new concentration using the standard formula.

Recalculate Your Dose in Seconds

Enter your dose in mg and your new vial's concentration. DoseCompa handles the math instantly — and supports auto-fill for major compounding pharmacies including Strive, Empower, Wells, Hallandale, and more.

Open the Calculator →

Frequently Asked Questions

Do I have to restart my GLP-1 titration when switching pharmacies?
No. Switching pharmacies does not reset your titration. Your dose in milligrams stays where it is. What changes is the unit count based on the new concentration. If your provider approves the switch, you continue at your current dose level and simply recalculate your units for the new vial.
How do I know what concentration my new pharmacy uses?
Check the label on your new vial. The concentration should be printed as mg/mL. If it is not explicit, look for the total mg and total mL and divide. You can also check the DoseCompa pharmacy database, which lists confirmed concentrations for major compounders.
My new vial has a much higher concentration than my old one. Is that safe?
Yes — as long as you recalculate your units accordingly. Higher concentration means you draw a smaller volume for the same dose. There is no clinical difference in how the medication works at different concentrations, provided the dose in milligrams is the same.
What if I accidentally injected the wrong dose from my new vial?
If you injected significantly more than intended (double dose or higher), contact your provider or a medical professional promptly. Common symptoms of excessive GLP-1 dose include severe nausea, vomiting, and in rare cases, low blood sugar. If you injected less than intended, it is generally not an emergency — monitor how you feel and discuss with your provider at your next check-in.
Can I mix vials from two different pharmacies?
No. Never mix the contents of two different vials. If you have leftover medication from your old pharmacy, complete that vial at the correct unit count before starting the new one. Mixing two vials with different concentrations makes accurate dosing impossible.

Sources

  1. FDA. Compounding and the FDA: Questions and Answers. Updated November 2024.
  2. Wilding JPH, et al. Once-Weekly Semaglutide in Adults with Overweight or Obesity. N Engl J Med. 2021;384:989-1002.
  3. Jastreboff AM, et al. Tirzepatide Once Weekly for the Treatment of Obesity. N Engl J Med. 2022;387:205-216.
  4. USP <797> Pharmaceutical Compounding - Sterile Preparations. United States Pharmacopeia.