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.
In the movie, the Grief Serum is in the form of a blue liquid, contained within a metal tube with an injector tip.