Botulinum toxin is a medication and a neurotoxic protein produced by the bacterium Clostridium botulinum. It is the most toxic protein known with an LD50 of roughly 0.005-0.05 µg/kg. Despite this, it is used in minute doses, in some parts of the world, to treat muscle spasms. It is sold commercially under the brand names Botox, Dysport, Myobloc, Neurobloc and Xeomin for this purpose. Botox Cosmetic and Vistabel are available for cosmetic treatment. The terms Botox (Cosmetic), Vistabel, Dysport, Myobloc, Neurobloc and Xeomin are trade names and are not used generically to describe the neurotoxins produced by C. botulinum.

From http://en.wikipedia.org/wiki/Botox#Medical_uses