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Botulinum toxin is a protein.
It can stop some of the chemical messages that are sent from nerves. Botulinum toxin is used to stop muscle spasms because it blocks these messages. Botulinum toxin is made by the same bacteria that causes food poisoning. A high dose of botulinum toxin could be fatal, just as food poisoning can be fatal. However, the dose given in injections is so small that it's unlikely you'll have any harmful effects from the toxin.