Botulinum toxin is nowadays approved as an effective medication for various neurological disorders. The extreme toxicity of this toxin-inducing botulism, a severe lethal muscle-paralyzing illness, has been well known since the seminal works of the end of the 19th century. Because of this toxicity, botulinum toxin was one of the first agents to be considered for use as a biological weapon. The Second World War was a crucial period for the first attempts to weaponize this toxin even if many unknown factors about botulinum toxin still existed at the outbreak of the war. Using documents from the British National Archives and other published sources, we discuss the main points of the attempts to weaponize this toxin in German and Allied armies. During WW2, Allied intelligence services regularly reported a major German threat related to the potential use of botulinum toxin as a biological weapon, especially during the preparation of <i>Operation Overlord</i>, the Allied invasion to liberate Europe. All these reports would ultimately prove to be inaccurate: botulinum toxin was not part of the German military arsenal even if some German scientists tried to use the results of the French pre-war military research. Misinformation spread by intelligence services stimulated military research at Porton Down facilities in England and at Camp Detrick in the USA. These studies led to a succession of failures and myths about the weaponization of botulinum toxin. Nevertheless, major progress (purification, toxoid) arose from the military research, providing useful data for the first steps of the therapeutic use of botulinum toxin in the post-war years.
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