The spread of injected botulinum toxin is commonly referred to as diffusion. Each of the 3 drugs mentioned: Botox, Xeomin, and Dysport each have different molecular weights that in theory should determine how far they can travel in the tissues. Xeomin is the lightest molecule, followed by Dysport and then Botox. Based on this data researchers have theorized that Xeomin should diffuse the easiest followed by Dysport then Botox. However, this has not been the case.