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 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.