Cersei Lannister acquires a bottle from Grand Maester Pycelle when readying for the Battle of the Blackwater. 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 A 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. "Nightshade" has also been mentioned as a poison.
- Sweetsleep at A Wiki of Ice and Fire (Major spoilers from the books)
- Nightshade at A Wiki of Ice and Fire