Poison

"- I heard it said that poison is a woman's weapon. - Yes. Women, cravens... and eunuchs."

- Ned Stark and Grand Maester Pycelle

A poison is a toxic substance that disrupts the proper functions of the body, often with lethal effects.

Poison is used throughout the known world, though is more prevalent in the Free Cities. In the martial society of Westeros poison is regarded with contempt and considered a weapon used only by cravens and women. Maesters study poisons at the Citadel.

Known poisons

 * Tears of Lys
 * Wolfsbane
 * Manticore venom
 * The strangler
 * Basilisk venom
 * Widow's Blood
 * Sweetsleep
 * Demon's Dance
 * Blind Eye

When Pycelle testifies at Tyrion's trial, he counts several of the poisons listed above.

Many strong medicines, such as Essence of Nightshade, can also be fatal if a high enough dosage is consumed.

The Crannogmen who live in the Neck coat their arrows with poisons obtained in the surrounding swamps.

Known Poisonings

 * Jon Arryn was suspected of having been poisoned. The truth emerged during a conversation between Arryn's widow Lysa and Petyr Baelish. Lysa reminded Baelish that she laced her ex-husband's wine with Tears of Lys, Baelish having told her to do it.
 * Strictly speaking, King Robert Baratheon was not poisoned, but his squire Lancel Lannister intentionally provided him wine which was spiked to be much more potent than he expected, to dull his wits during a dangerous boar hunt. This was arranged by Lancel's cousin, Robert's wife Queen Cersei Lannister. The plan succeeded when a large boar attacked Robert but he did not react quickly enough to stop it from goring him with its tusks, mortally wounding him. Given that Robert was a well-known drunk, and no actual "poison" was used, this made it more difficult to suspect foul play.
 * Daenerys Targaryen has twice been targeted with poison. The first occasion was when a wineseller on Robert Baratheon's payroll attempted to give her poisoned wine. Ser Jorah Mormont realized what was about to occur and foiled the attempt. The second was organized by the Warlocks of Qarth, using a manticore hidden in a wooden sphere. The creature was killed by Ser Barristan Selmy before it could sting her.
 * Maester Cressen tried to poison Melisandre during a toast. At his behest, they both drank the wine he had spiked with the strangler. He collapsed and died, but the poison had no effect on her.
 * At Harrenhal, Jaqen H'ghar murdered Ser Amory Lorch with a dart dipped in wolfsbane.
 * King Joffrey Baratheon was poisoned at his wedding feast with the strangler, which had been put into his wine goblet. His uncle Tyrion Lannister was put on trial for the crime, though it was actually orchestrated by Petyr Baelish and Olenna Tyrell.
 * Ser Gregor Clegane is badly wounded in his ultimately victorious duel with Oberyn Martell. It was later discovered by Grand Maester Pycelle and Qyburn that before the duel Oberyn laced his spear with manticore venom, causing Clegane's wounds to horrifically putrefy and leaving him in terrible agony.

In the books
In the A Song of Ice and Fire novels, there is no specific medicine called "Essence of Nightshade": in the TV series, it appears to stand in for a variety of analgesic or anesthetic medicines which were present in the novels. "Essence of Nightshade" actually takes the place of a similar medicine in the novels, "Sweetsleep". The TV series started referring to "Essence of Nightshade" in Season 2, but then actually mentioned Sweetsleep in Season 4.