If a 100‑unit onabotulinumtoxinA vial is reconstituted with 2.5 mL bacteriostatic saline and a 1‑mL syringe calibrated to 100 units per mL is used, how many units correspond to the marking “4” on the syringe?

The vial you reconstituted contains 100 U of botulinum toxin diluted with 2.5 mL of bacteriostatic saline. That creates a concentration of 100 U ÷ 2.5 mL = 40 U per milliliter. The syringe you are using is calibrated so that the full 1 mL volume corresponds to 100 U on the scale (i.e., 1 mL = 100 U on the syringe). Therefore each unit marking on the syringe represents 0.01 mL (1 mL ÷ 100). If you draw “4” on the syringe, you are drawing 0.04 mL of the diluted solution. At 40 U per mL, 0.04 mL contains 0.04 mL × 40 U/mL ≈ 1.6 U of botulinum toxin. In other words, “4” on that syringe delivers roughly 1½–2 U of Botox. Be sure to verify the concentration on the vial label, confirm the syringe calibration, and double‑check your calculations before injecting, especially if the intended clinical dose is critical. If a higher or more precise dose is required, consider using a syringe calibrated in 0.1‑mL increments or a dedicated Botox dilution protocol to avoid dosing errors.