The Grief Serum is the antidote that the Gladers give to someone who is stung by a Griever. It arrives in a syringe with their weekly supplies. After someone has taken the Grief Serum, they go through a process called the Changing. During the Changing, the patient goes through extreme pain, as well as regaining some of the memories of their past life. A person who is stung by a Griever and does not receive the Grief Serum dies.
It was mentioned in the Maze Runner Files that the Griever's sting gives the Gladers a mutated version of the Flare that can be cured with the serum. The serum has cured multiple major characters: Thomas, Gally, and Alby.
In the movie, the Grief Serum is in the form of a blue liquid, contained within a metal tube with an injector tip.