Botulinum toxin is a neurotoxic protein produced by the bacterium Clostridium botulinum. It is the most poisonous naturally occurring substance in the world. Though it is highly toxic, it is used in minute doses both to treat painful muscle spasms, and as a cosmetic treatment in some parts of the world. It is sold commercially under the brand names Botox and Dysport for this purpose. The terms Botox and Dysport are trade names and are not used generically to describe the neurotoxins produced by Clostridia species.
-- From Wikipedia