From Tar Valon Library
The herb called forkroot is known to be used to block someone's ability to channel . You can render someone who can channel totally unconscious with it (KoD, Prologue). Used in the right way it can make your limbs useless and leave you aware of your surroundings but totally useless. It is minty in flavor so its easily hidden in tea. Its effects were discovered by seamstress in Mardecin named Ronde Macura who was known to be one of the eyes and ears of the Yellow Ajah (TFoH, Ch. 10). She reported her findings of its uses to the White Tower, where it was not well received as it seemed to strongly targeted at those who could channel (ACoS, Prologue). The Seanchan use forkroot to detect male and female channelers and capture them (KoD, Ch. 4).
Its effects have become well known to almost everyone in the tower now. There are other herbs known to overcome the forkroot. In Winters Heart Nynaeve gives Elayne a dose of Greenwort and goatstongue to overcome some of the effects of the forkroot (WH, Ch. 10).
- Ronde Macura subdues Elayne and Nynaeve with Forkroot (TFoH, Ch. 10).
- Nynaeve gives Moghedien forkroot in Tel'aran'rhiod (TFoH, Ch. 55).
- Elayne is given forkroot in the fake assasination attempt (WH, Ch. 10).
- Teslyn gives Jolene forkroot to prevent her from interfering with Elayne and Nynaeve (WH, Ch. 19).
- Egwene is given forkroot when captured by the White Tower Aes Sedai (KoD, Prologue).
- Perrin doses the town of Malden's water supply with forkroot (KoD, Ch. 26).
Forkroot was not lethal — at least you woke, if you drank enough to sleep — but a tea that deadened a woman's ability to channel seemed aimed too directly at Aes Sedai (ACoS, Prologue).