Botulinum toxin units are not interchangeable
Each preparation is measured by its own manufacturer's assay, no conversion between systems is licensed, and volume markings on a syringe are not potency units.
Every botulinum toxin preparation licensed in the UK is measured in units, and none of those units mean the same thing.
Why the systems differ
Potency is not a mass. These products are not dosed by micrograms of protein; they are dosed by biological activity, established by each manufacturer’s own assay under that manufacturer’s own conditions.
Different assay, different reference standard, different unit. Allergan Units, Speywood units and the unit systems of other preparations are each internally consistent and mutually untranslatable.
The SmPCs say this explicitly. The Botox product information states that botulinum toxin units are not interchangeable from one product to another; the Azzalure information says the same of Speywood units. No conversion factor between products is licensed anywhere.
Why ratios circulate anyway
Because practitioners move between products and want a rule of thumb. Ratios appear in training material, in conference talks and in conversation, and different sources give different numbers.
None of them is approved. Using one means dosing a prescription-only medicine from an unapproved source, and the discrepancy between sources is itself the evidence that no reliable single figure exists.
The correct reference for any product is that product’s own SmPC, current version.
Volume is not potency
A separate confusion, and a more dangerous one: whether product units can be read off the graduations of an insulin syringe.
An insulin syringe is graduated in volume. Its marks tell you how much liquid is in the barrel. Potency units measure biological activity. The relationship between a volume marking and a number of potency units depends entirely on the concentration achieved at reconstitution, which is set by how the vial was prepared.
Two quantities, two meanings. Treating a syringe marking as a potency figure is a dosing error.
What follows for practice
Know which product is in front of you. Read that product’s current SmPC rather than a remembered figure. Do not carry a dose across from another preparation. And treat any conversion ratio offered without a licensed source as what it is — an unverified number applied to a prescription-only medicine.
This page is a reference note. It carries no dose, no injection points and no dilution figures: those are set by the manufacturer and published in the Summary of Product Characteristics, which is linked below. This site sells nothing and is not a clinic.
