The poison mechanic was introduced in Legends with Pit Scorpion and Serpent Generator, where it was defined as "Whenever this creature deals damage to a player, that player gets a poison counter." (A player with ten or more poison counters loses the game.) The mechanic appeared in sets up until Visions but was never strongly supported.  Poison was considered for the unreleased Unglued 2 set.  The mechanic then resurfaced briefly in Future Sight as poisonous, which made it possible to give multiple poison counters at once. At last poison became a major theme in the entire Scars of Mirrodin block with infect. Now poison counters were not given when a creature dealt damage to a player, but instead of dealing damage to a player.  Mirrodin Besieged added the concept of a player being “poisoned” if he or she has one or more poison counters, and cards that would benefit from opponents being "poisoned".  The block provided three different cardboard versions for poison counters, as part of the marketing cards. 
In Two-Headed Giant, a team with fifteen (15) poison counters loses the game.
Since the counters have no other properties, and rarely any other card affects or is affected by them, they do not hold much power in the metagame. Two keyword abilities use poison counters to inflict damage: "Infect", and "Poisonous". With infect; how many poison counters a creature deals to an opponent is equal to its power: 1/1 = deals 1 poison counter. 3/2 = deals 3 poison counters. These are dealt instead of regular damage, not in addition to it. However, with "Poisonous", the number of poison counters dealt in damage is equal to the N value in Poisonous N. For example, a 1/1 creature with Poisonous 2 deals 2 poison counters, as well as the 1 (non-poison) Damage, from the creature's power.
Cards using poison counter(s) before the keyword Infect & Poisonous
- Crypt Cobra
- Marsh Viper
- Pit Scorpion
- Sabertooth Cobra
- Serpent Generator which creates Snake artifact creatures
- Suq'Ata Assassin
- Swamp Mosquito
- Mark Rosewater. (November 24, 2014.) "Top 8 and a Half Tales", Daily MTG, magicthegathering.com, Wizards of the Coast.
- Magic Arcana. (June 27, 2005.) "Mad Beetdown!", Daily MTG, magicthegathering.com, Wizards of the Coast.
- Mark Rosewater. (September 06, 2010.) "Something Wicked This Way Comes, Part 1", Daily MTG, magicthegathering.com, Wizards of the Coast.
- Wizards of the Coast. (January 2011.) "Mirrodin Besieged mechanics", Daily MTG, magicthegathering.com, Wizards of the Coast.
- Monty Ashley. (April 20, 2011.) "The Changing Face of Poison", Daily MTG, magicthegathering.com, Wizards of the Coast.