Wolfsbane is a poison.
A Clash of Kings
Tyrion Lannister sees wolfsbane in Grand Maester Pycelle's collection of medicines and poisons.
A Storm of Swords
During Tyrion Lannister's trial for the death of King Joffrey Baratheon, Grand Maester Pycelle claims that Tyrion stole a sample of wolfsbane as well as other drugs from Pycelle's collection.