Titan injection (Anime)

History
The origin of Titan injections and their usage is unknown.

In the year 785, a cult was discovered by a force of soldiers, and the man who named Ymir 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 transformed into Titans by the use of a serum.

The Fall of Shiganshina arc
On the night of the fall of Shiganshina, Grusha Jaeger reunited with his son, Eren, and injected him with a serum, enabling him to transforming him into a Titan.