Essence of Nightshade

Essence of Nightshade is a poisonous substance. It can be formulated into a potion used as an anxiolytic in small doses and a poison at higher doses. Cersei Lannister acquires a bottle from Grand Maester Pycelle when readying for the Battle of the Blackwater. He informs her that a single drop will calm frayed nerves, 3 drops will put a person into a deep, dreamless sleep, and warns that 10 drops will kill. Cersei almost uses the poison on Tommen Baratheon, to prevent him from being captured when the battle seems lost. When she is halted by the arrival of Tywin Lannister to announce victory she drops the bottle to the floor, smashing it.

In the books
In the Song of Ice and Fire novels there is a medicine called Sweetsleep which has slightly similar properties. It is used as an anticonvulsant and anxiolytic at low doses.