Which is why the amount of units required to achieve the same clinical effect and duration matters.
This is why Dysport initially struggled in the US. While it had been used for years in europe, essentially at a 3:1 unit dose ratio to Botox with nearly identical clinical effect and duration (albeit with slightly faster onset and slightly longer duration), when initially approved in the US it was approved with dosing equivalent to approximately 2.5:1 SU (Speywood U) to IU (International U). My European colleagues laughed at this as they had been dosing at a 3:1 SU/IU ratio for years. The Dysport vial was even 300u which made it essentially interchangable with Botox if reconstituted the same (ie 1ml per vial, 2ml per vial), as the units were essentially 3:1 for the same drawn volume.
Allergan used this as a lynchpin in their 'toxins are not interchangable' campaign when dysport came out, and it took several years to correct not only the confusion over this but also the fact that a skilled injector can use these interchangeably rather easily. The 'dysport diffusion' knock continued for a while, until people realized that this was actually advantageous rather than detrimental.
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