If you get poisoned by a poisonous potato it'll give you "Poison" for 5 seconds.
Eating one of these at night is very risky, as it'll fill the hunger bar but deplete your health. If you come face to face with something like a skeleton or zombie , you will have a higher chance of death than you normally would.
You can tell if a potato is poisonous because normal potatoes do not have green around them. Another way to tell is that it will not stack with regular potatoes.
Another way to tell is that it will not stack with regular potatoes.