Some spells, such as the Snake Bite (Snake Magic), give you a chance of Poisoning a monster.

A monster is Poisoned when it turns green. A green poison vial and a turn counter will appear in front of it. The counter indicates how many card turns remain before the Poisoning effect wears off. 

In the example below, the Frog is Poisoned and the counter shows 3, meaning the Frog will remain Poisoned for 3 card turns, including the current one.

Note: You can Poison any monster of your choice. Smaller monsters, such as Baby Ghost and Frog, are more susceptible to being Poisoned.

Learn more about: Challenge - Defeat X Poisoned monsters