Nightshade is a poison. It is one of the more common poisons.
A Clash of Kings
Tyrion Lannister sees nightshade 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 nightshade as well as other drugs from Pycelle's collection.