Poison (毒) is a status condition that exists in the Fire Emblem series. There are very few status conditions that a character can be inflicted with in the series, such as Poison, Sleep, Silence and Berserk. Most of the other status problem are caused by staves that only have the ability to inflict an opponent with a condition, however, Poison is usually inflicted by certain weapons that deal damage as well.
When inflicted to a unit (be it player controlled, enemy, or any other unit) poison will inflict damage to a unit at the beginning of their turn. In most of the Fire Emblem games the amount of damage is random and ranges between 1 and 5. While poisoned a unit can not be inflicted with a second status condition.
Poison will wear off after a certain amount of turns in most games. In the majority of the games it will cure itself in only 5 short turns, within that time it has the potential to do at least 5 damage and up to 25 damage total. All considered that isn't all that much damage, and over five turns allows for lots of time to heal. However, in Thracia 776, all status conditions last the whole map, including poison. Adding that to the fact that poison is so common with Dark Mages equipped with Yotsmungand everywhere, Poison can be lethal, as without being attended to by a healer, Vulnerary, Antitoxin, or the Restore staff, the player could be in a lot of trouble.
Poison is considered by many to be the worst of all the Fire Emblem series status conditions. Other conditions mainly prevent the enemy from attacking while poison does small amounts of damage. Poison is also the most easily cured of the status conditions. Not only can it be cured by the Restore staff and other methods that cure all status problems but there is an item, the Antitoxin, that works specifically to cure poison.
Poison is also one of the hardest status conditions to be inflicted by the player, due to the low amount of weapons available for poison without codes. The Poison weapons were enemy-exclusive in FE5-9, and they must be stolen via Disarm in FE10, and they are, however, only seen in Part 2 (Part 1 has Poison weapons, but lacks Disarm). The only weapon that can poison without being stolen is Valaura in Radiant Dawn. However, Valaura is S-Rank Light magic that you get near the end of the game, making it not that useful overall.
Yotsmungand can technically be stolen, but it can only be wielded by one playable character and it only poisons when used by the enemy, not the player. In Thracia 776, none of the Poison weapons even inflict poison on enemy units due to how the game is programmed. In order for players to ever use poison against the enemy, they must hack (or use the Enemy control glitch in Fire Emblem: The Blazing Blade).
Poison is caused by certain weapons. In most games they are caused by the Venin weapons (Known as Poison weapons in some games). These include the Venin Edge, Venin Lance, Venin Axe and Venin Bow. Venin weapons cannot usually be bought in stores during the games, however, they can be stolen from some enemies that wield them. When a Venin weapon hits a targeted character they are automatically poisoned, however, Venin weapons have lower accuracy, and deal lower damage than even Iron weapons.
In some Fire Emblem games certain enemies can cause Poison with special weapons that are unique to that enemy. An example of this can be found in Fire Emblem: The Sacred Stones. In this game, about half the enemies are monsters. Some of these monsters use claws, teeth, and other built in weapons to fight. A few of these monster's built in weapons cause poison. Such examples include the Bael who may use Poison or Fetid Claws (both are capable of poisoning), and weapon-using monsters carrying such weapons like poison swords and lances. Even if the monster is incapable of dealing damage to the target, they can still inflict poison on them if it hits them. Poison can be cured in The Sacred Stones through the Latona/Restore staves and Antidotes.
Poison is still often left out of any player strategies due to the fact it is often easier to simply kill an enemy with conventional weapons.