If you're using compounded GLP-1 medications from a multi-dose vial, you'll need to do some basic math to ensure you're injecting the correct dose. This guide will walk you through the calculations step by step—no advanced math required.
Why This Matters
The FDA has received reports of patients accidentally injecting 5 to 20 times their intended dose due to confusion between units, milligrams, and milliliters. These errors led to hospitalizations for severe nausea, vomiting, dehydration, pancreatitis, and dangerously low blood sugar. Taking a few minutes to understand the math could prevent a serious medical emergency.
The Three Terms You Need to Know
Before we get into calculations, let's clarify three terms that often cause confusion:
Milligrams (mg) — Your Dose
This is the amount of medication you're prescribed. When your provider says "take 0.5 mg weekly," they're telling you how many milligrams of semaglutide or tirzepatide to inject. This is your target.
Milliliters (mL) — The Volume
This is the volume of liquid you'll draw into your syringe. One milliliter equals 100 "units" on an insulin syringe. The liquid contains the medication dissolved within it.
Concentration (mg/mL) — The Key Number
This tells you how much medication is dissolved in each milliliter of liquid. If your vial says "5 mg/mL," that means every 1 mL of liquid contains 5 mg of medication. This is the number you need to check on your vial label.
The Basic Formula
Here's the formula that makes everything work:
Your Dose (mg) ÷ Concentration (mg/mL) = Volume to Inject (mL)
Then multiply by 100 to get units on an insulin syringe
Let's see this in action with real examples.
Semaglutide Dose Calculations
Compounded semaglutide commonly comes in concentrations of 2.5 mg/mL or 5 mg/mL. Here's how to calculate doses for each:
Example 1: 5 mg/mL Concentration
Your vial says "Semaglutide 5 mg/mL" and you need to inject 0.25 mg (the starter dose).
Step 1: 0.25 mg ÷ 5 mg/mL = 0.05 mL
Step 2: 0.05 mL × 100 = 5 units
Draw up to the "5" mark on your insulin syringe.
Example 2: 2.5 mg/mL Concentration
Your vial says "Semaglutide 2.5 mg/mL" and you need to inject 0.5 mg.
Step 1: 0.5 mg ÷ 2.5 mg/mL = 0.2 mL
Step 2: 0.2 mL × 100 = 20 units
Draw up to the "20" mark on your insulin syringe.
Full Semaglutide Reference Table (5 mg/mL)
| Dose (mg) | Volume (mL) | Syringe Units |
|---|---|---|
| 0.25 mg | 0.05 mL | 5 units |
| 0.5 mg | 0.1 mL | 10 units |
| 1.0 mg | 0.2 mL | 20 units |
| 1.7 mg | 0.34 mL | 34 units |
| 2.4 mg | 0.48 mL | 48 units |
Tirzepatide Dose Calculations
Tirzepatide often comes in concentrations of 10 mg/mL or higher (some pharmacies compound at 15 or 20 mg/mL for higher doses). Always check your specific vial.
Example: 10 mg/mL Concentration
Your vial says "Tirzepatide 10 mg/mL" and you need to inject 2.5 mg (the starter dose).
Step 1: 2.5 mg ÷ 10 mg/mL = 0.25 mL
Step 2: 0.25 mL × 100 = 25 units
Draw up to the "25" mark on your insulin syringe.
Full Tirzepatide Reference Table (10 mg/mL)
| Dose (mg) | Volume (mL) | Syringe Units |
|---|---|---|
| 2.5 mg | 0.25 mL | 25 units |
| 5.0 mg | 0.5 mL | 50 units |
| 7.5 mg | 0.75 mL | 75 units |
| 10 mg | 1.0 mL | 100 units (full syringe) |
Higher Doses May Require Different Syringes
At tirzepatide doses of 10 mg and above with a 10 mg/mL concentration, you may need to use a larger syringe or a higher concentration medication. Some pharmacies compound at 15 or 20 mg/mL specifically to keep injection volumes manageable. Always confirm with your provider.
Common Mistakes to Avoid
Mistake #1: Confusing "Units" with "Milligrams"
On an insulin syringe, the numbers represent volume (units = 1/100th of a mL), NOT milligrams of medication. If you're told to take "0.5 mg" and you draw to "50 units," you've overdosed by 10x if your concentration is 5 mg/mL.
Mistake #2: Not Checking the Concentration
Compounding pharmacies use different concentrations. A dose of 10 units from a 2.5 mg/mL vial delivers a completely different amount than 10 units from a 5 mg/mL vial. Always read the label on YOUR specific vial.
Mistake #3: Assuming Instructions Are Universal
If you find dosing instructions online or from another patient, they may not apply to your medication. Concentrations vary. Always calculate based on your specific vial and prescription.
Mistake #4: Using the Wrong Syringe
Most compounded GLP-1s are designed to be measured with U-100 insulin syringes (where 100 units = 1 mL). If you're using a different syringe type, your measurements could be completely wrong.
What to Do If You Make a Dosing Error
If you suspect you've injected more than intended:
- Don't panic—but do act
- Contact your healthcare provider immediately
- Monitor for symptoms: severe nausea, vomiting, dizziness, sweating, confusion
- If symptoms are severe, go to the emergency room
- Keep the vial handy so medical staff can see the concentration
Symptoms of overdose include severe gastrointestinal distress and potentially dangerous low blood sugar (hypoglycemia). Because semaglutide has a half-life of about one week, symptoms may persist and require extended monitoring.
Tips for Safe Dosing
- Double-check every time: Before you inject, verify your calculation. Every time.
- Use good lighting: Read syringe markings in bright light, especially for small doses.
- Write it down: Keep a dosing chart with your exact units for each dose level.
- Ask your provider for verification: When starting or changing doses, confirm the calculation with your prescriber.
- Store your chart with your supplies: Keep a laminated card or sticky note with your current dosing information right next to your vials.
Pro Tip: Request Pre-Filled Syringes
Some compounding pharmacies offer pre-filled syringes, which eliminate the need for calculations entirely. This costs more but provides peace of mind. Ask your provider if this option is available.
The Bottom Line
The math isn't complicated once you understand it, but the stakes are high enough that you should never guess. Before every injection, know your prescribed dose (mg), check your vial's concentration (mg/mL), do the division, and verify the number of units you're drawing. A few extra seconds of calculation is a small price to pay for safety and peace of mind.
If you ever feel uncertain about your dosing, stop and contact your healthcare provider before injecting. There's no such thing as being too careful when it comes to medication dosing.
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