There is, quite literally a 50% chance of being poisoned by the possibly-rotten pizza.

On success, the following message appears and a random amount (between 10 and 50) pizzas is eaten:

Nom.. nom..
You eat the pizzas, luckily they are not rotten

On failure, you are poisoned, giving various negative effects throughout the game. The poison can be removed, as per the message that appears:

Nom.. nom..
You eat the pizzas, but... they are rotten!
You are poisoned, this causes you to die easily in battles even if you have a lot of hp
Luckily you can heal yourself by drinking 10 health potions [Drink 10 health potions]

There is, quite literally a 50% chance of the pizza being rotten.

On success of that random event, the following message appears and a random amount (between 10 and 50) pizzas is eaten:

Nom.. nom..
You eat the pizzas, luckily they are not rotten

On failure, you are poisoned, giving various negative effects throughout the game. The poison can be removed, as per the message that appears:

Nom.. nom..
You eat the pizzas, but... they are rotten!
You are poisoned, this causes you to die easily in battles even if you have a lot of hp
Luckily you can heal yourself by drinking 10 health potions [Drink 10 health potions]