The serum is used to turn humans into Titans shortly after they are injected followed by a sudden flash of light.
Over sixty years ago, a cult was discovered by a force of authorities, and the man who gave "Ymir" her name claimed she had deceived him. Ymir chose to abide by her given name hoping to save the other members of the cult, but this proved fruitless and she was arrested with the other cultists. After being paraded around town and stoned by angry citizens, the cultists were taken to a remote borderline. Despite Ymir's pleas for mercy, she and the other cultists were transformed into Titans by the use of an injection.
The Reiss family used the Titan injection to pass down the power of the Founding Titan through the generations. This practice had existed since the raising of the Walls. It saw Uri Reiss inherit the power from his father and, in 842, Frieda Reiss inherit it from him in turn.
After the fall of Wall Maria, Eren briefly sees visions of his father Grisha Jaeger forcing him to receive some sort of injection. The injection turned Eren into a Titan who then devoured Grisha.
When cornered by the Trost District Garrison concerning his newfound Titan power, Eren suddenly recalls the moment when his father met him in the forest after the fall of Shiganshina. Insisting that Eren must return to the basement of their home, Grisha gave Eren an injection which he said would damage his memory and grant him a new ability. Furthermore, his father claimed that "their memories" would help Eren in learning how to use his new power.
In the Underground Chapel, Rod Reiss prepares to use an injection on his illegitimate daughter Historia. Upon seeing the syringe for the Titan injection, Eren begins wildly struggling against his chains, recognizing it as the kind his father had given him five years ago. Rod attempts to influence Historia into taking the injection and devouring Eren, however, after much contemplation, Historia rejects this plan and shatters the syringe. As she begins freeing Eren from his chains, Rod desperately licks the exposed serum, becoming a monstrous Pure Titan.
Levi later finds a heavily wounded Kenny Ackermann leaning against a tree and observes the man's injuries have left him on the verge of death. Kenny responds by smugly revealing that he has a Titan injection, which could allow him to save himself. However, Kenny does not use it on himself. As a final gift, Kenny decides to give Levi the Titan injection.