The 100‑unit vial of onabotulinumtoxinA should be reconstituted with 2.5 mL of preservative‑free normal saline. This gives a final concentration of 40 units per millilitre (100 U ÷ 2.5 mL = 40 U/mL).
Insulin pen needles are calibrated for insulin, delivering 100 units of insulin per millilitre, which means that each “unit” on the pen corresponds to 0.01 mL of fluid. Because the Botox solution contains 40 U in each millilitre, every 0.01 mL (one insulin‑pen unit) contains 0.4 units of Botox (0.01 mL × 40 U/mL = 0.4 U).
Consequently, to give a typical Botox dose you would need to set the pen to a fractional number of insulin units (e.g., a 5‑unit Botox dose would require about 12.5 insulin‑pen units). Most insulin pens cannot be set to half‑units reliably, making accurate dosing difficult and increasing the risk of under‑ or over‑dosing.
Recommendation: Do not use insulin pen needles for Botox administration. Instead, draw the reconstituted toxin with a standard 1‑mL tuberculin (or 0.5‑mL) syringe and a 30‑ to 31‑gauge needle. This allows precise volume measurement (e.g., 0.125 mL for a 5‑unit dose, 0.250 mL for a 10‑unit dose, etc.) and reduces the chance of dosing error.
If a pen‑type device must be used, ensure the device can be set in 0.5‑unit increments and calculate the required setting by dividing the desired Botox units by 0.4 (e.g., desired 8 U ÷ 0.4 = 20 insulin‑pen units). However, this practice is not standard and is not recommended for safety and accuracy.
Second‑line option: If a syringe is unavailable, a 2‑mL syringe with a fine needle can be used, but the same principle of calculating volume (desired Botox units ÷ 40 U/mL) applies. Always observe aseptic technique, label the prepared solution, and use it within the recommended time frame (typically 4 hours after reconstitution).