An unusual adverse event of botulinum toxin injection in the lower face.

Although treatments with botulinum toxin A are considered safe, unexpected results can occur. In our case, it probably occurred due to medial injection or diffusion of botulinum toxin A into depressor anguli oris muscle on the left side that may lead to paralysis of the left depressor labii inferioris muscle and downward hyperfunction of the unaffected right lower lip. The correction was made with a new injection of botulinum toxin A in the depressor labii inferioris hyperactive muscle at the right side.