With their article in this issue, Hexsel et al1 have made an important and challenging observation. They injected each side of the frontalis muscle with equal unit doses of 2 of the commercially available botulinum toxins—units that most scientists who understand the methods of unit measurement, as well as clinicians who use the toxins, would say are not equal. Their finding that the diffusion halo of onabotulinumtoxinA is greater than the diffusion halo of abobotulinumtoxinA when measured by anhidrosis would seem to confirm that the onabotulinumtoxinA units are more potent than the abobotulinumtoxinA units. However, in their conclusion, the authors focus on the fact that there is no difference between the muscle effects of the 2 products as measured by compound muscle action potential or clinical wrinkle effect. So does this suggest that we need to rethink our view of these “units”? Does it mean that the authors have teased out the separate effects of these 2 neuromodulators on striated muscle and glandular function, or does it confirm a difference in potency?
Carruthers J, Carruthers A. Are All Units Created Equal?. JAMA Dermatol. 2013;149(12):1377. doi:10.1001/jamadermatol.2013.6836