Charcot, the great 19th Century French neurologist discovered the anti-Parkinson actions of belladonna. However, in dilating their pupils, their vision became blurred. At a reception, a Parisian beauty may have inadvertently bumped into Charcot. Good manners precluded her from mentioning constipation. Because anti-cholinergic drugs are small molecules they diffuse from their site of administration, gain access to the circulation and appear everywhere.
Botulinum toxin is a large protein molecule that blocks acetyl-choline receptors.
Thus the effects of botulinum toxin remain confined to it's site of injection. Botulism, unrecognized and untreated, is fatal. The purified toxin, when properly administered by a trained specialist is safe. The purified toxin doesn't diffuse from it's injection site. And, as there no living Clostridum botulinum bacteria to multiply, there's nothing to produce more toxin.