The invention relates to a modified botulinum toxin comprising a natural heavy chain and a modified light chain, characterized in that the modification of the light chain resides in that it comprises (i) an extension of the chain on its N-terminus which has the structure -(C)n-(tag)m-(X)l- in the direction from the N- to the C-terminal end, whereinC represents a cysteine residue,tag represents any tag andX represents the residue of any naturally occurring amino acid,n represents an integer from 1 to 50,m represents 0 or 1, andl represents 0 or an integer from 1 to 50,and in that (ii) at least one of the cysteine residues in the extension of the chain is coupled to at least one chain of PEG.
