What toxin gives hemlock water-dropwort its deadly poisonous effect?

The toxin that gives hemlock water-dropwort its deadly poisonous effect is oenanthotoxin. It is produced by Oenanthe crocata, the species commonly called hemlock water-dropwort, and is one of the plant’s principal neurotoxic compounds.

Oenanthotoxin is a highly unsaturated polyacetylene rather than an alkaloid. The roots are considered the most poisonous part, although the whole plant can contain dangerous toxins. Its concentration varies with geography and season, with levels often highest in late winter and early spring. Unlike many poisonous plants, water-dropwort may have a pleasant smell and sweet taste, making accidental ingestion especially hazardous.

A frequent mix-up is with cicutoxin. Cicutoxin is chiefly associated with water hemlocks in the genus Cicuta, not with hemlock water-dropwort in Oenanthe. The names sound similar because both plants belong to the carrot family, Apiaceae, but they are different genera and have different signature toxins. Oenanthotoxin and related compounds affect the nervous system and can produce severe convulsions and rapid poisoning.