Poison

Poison (sometimes known as Sick) is an Extra Rule that allows players to leave the square in order to to do something of relative urgency. Without risking getting out. It is required to be called out

Uses
Most often the rule is used while returning to a lower ranked square, leaving the square to move something, or to continue eating (schoolyard specifc)

Another use of Poison, is to get someone out. Due to the poisoned square still existing among the other squares, it is common for the Ball to be played into the poisoned square. This causes the last player to touch the ball to be out.

Many players choose to call out Poison, just as the Ball is about to land in their square.

Antedote
Antedote (sometimes called Doctor or Medical) is called out to reverse the effects of Poison

Antedote Sweep
Antedote Sweep (known as Full Antedote in some locations), is used by Ace to reverse all Poison squares.