Potent neurotoxic alkaloids made by freshwater cyanobacteria, chiefly anatoxin-a from Anabaena, Oscillatoria and related genera. They act as agonists of nicotinic acetylcholine receptors, overstimulating muscles until twitching, convulsions and respiratory paralysis set in; livestock, pets and wildlife drinking from affected waters can die within minutes — the origin of the historical name ‘very fast death factor’. Human poisonings are rare, but water utilities monitor for anatoxins during bloom seasons alongside the better-known microcystins. Control means managing nutrient runoff that feeds blooms and treating affected water with activated carbon or oxidation.