GLP-1 Vial Transition Calculator

Why concentration transitions are dangerous

The single most common dosing error in compounded GLP-1 therapy happens when the vial concentration changes and the patient does not adjust their syringe units. Because the formula is:

units = (dose in mg × 100) ÷ concentration in mg/mL

...a higher concentration means fewer units for the same mg dose. If the patient keeps drawing the old number of units from a higher-concentration vial, they will inject a higher mg dose — potentially double or more [1].

Example: semaglutide 2.5 → 5 mg/mL

A patient on 1.0 mg/week semaglutide was drawing 40 units from a 2.5 mg/mL vial. The pharmacy switches to a 5 mg/mL vial. If the patient keeps drawing 40 units:

(40 × 5) ÷ 100 = 2.0 mg — double the intended dose.

The correct new reading is:

(1.0 × 100) ÷ 5 = 20 units — half the old reading.

This calculator does that math for you and warns you when the change is large enough to be high-risk (more than 2× change) or when the new unit count exceeds the syringe capacity (100 units for a standard U-100 insulin syringe) [1].

Common concentration transitions

• Semaglutide 2.5 → 5 mg/mL: Patient was on a low-concentration vial and the pharmacy switches to a more concentrated formulation. Units are halved.
• Semaglutide 5 → 10 mg/mL: High-dose patients (1.7-2.4 mg) on 5 mg/mL sometimes get switched to 10 mg/mL to reduce injection volume. Units are halved.
• Tirzepatide 10 → 20 mg/mL: Higher-dose tirzepatide patients (10-15 mg) may switch to 20 mg/mL to keep the syringe reading under 100 units. Units are halved.
• Any concentration decrease: If the new concentration is lower than the old one, the patient must draw more units. If the new reading exceeds 100 units, they will need to split into two draws.

Important safety disclaimer

This tool performs arithmetic only and does not provide medical advice. Every time you receive a new vial, verify the concentration on the label before drawing any dose. If the concentration is different from your previous vial, recalculate your units using this tool and confirm with your prescriber or compounding pharmacy before injecting.

Related tools and research

• GLP-1 unit ↔ mg converter — convert between milligrams and syringe units at any concentration.
• GLP-1 reverse dose calculator — verify the mg dose from your syringe units.
• GLP-1 split dose calculator — split your weekly dose into smaller injections.
• GLP-1 reconstitution calculator — calculate BAC water volume for lyophilized vials.
• GLP-1 dose plotter — visualize plasma concentration at each titration step.
• Compounded semaglutide bioequivalence — quality testing requirements for 503A/503B pharmacies.