Botulinum toxin A, molecular model. This highly toxic protein is produced by the bacterium Clostridium botulinum. It is one of the most toxic substances known. It has its effect by binding to the junctions between muscles and nerves and preventing the release of the signalling molecule acetylcholine. This causes paralysis of the muscles, which leads to death through respiratory failure. Its action on muscles has led to its use in medicine, mainly in cosmetic procedures, where tiny doses are injected into overactive muscles. This relaxing of muscles has the effect of smoothing wrinkles, and it is widely used for that purpose under the brand names Botox and Dysport.