You want to poison a player? Use a "Splashpotion of Poison
You use the Splashpotion like in vanilla minecraft with a simple "rightclick"
The duration of effect is maximal 33 seconds. In this time the player lose constantly life, but the potion can't kill a player.
You can get the potion at the secretshop for 150$ apiece.
The ID is (373:1638)
It is not easy to use splashpotions. Use them only when you are sure that you hit the player.
You can get posioned by your own potion!