The injection only works on Subjects of Ymir and has no effect on other living things. While the exact chemical composition of Titan serum is currently unknown, it is considered by the Marleyans to be Titan spinal fluid. It has been shown that the serums quickly evaporate after coming into contact with air; for this reason, they are always kept in tightly-sealed vials. Since Titan corpses also quickly evaporate, it would make sense that their spinal fluid would do the same.
Rod Reiss had with him a "strongest" and an "armor" serum along with the regular serum. It is unknown if there are more types. The "powerful" serum, when ingested orally, turned Rod into a 120m Titan, which Kenny Ackermann theorized that he did not follow the process correctly, and the "armor" serum gave Eren Jaeger's Titan hardening abilities.
Titan serums are injected into the bloodstream, typically the inner forearm or the back of the neck; it can also be ingested. Very shortly after it is introduced into the bloodstream, the person generates a Titan body and becomes a "Pure Titan." Pure Titans are mindless until they consume a human with the power of the Titans. Humans with the power of the Titans can take the serum again, and specialized serums can give the Nine Titans abilities they never had; it is unknown what would happen if they take the normal serum. After witnessing Rod Reiss turn into a disfigured 120m Titan when he licked the "strongest" serum, Kenny speculates that the method in which the serum enters the body matters, and if done improperly, there may be consequences. The size of the resulting Titan also appears to be controllable, as Sergeant Major Gross claimed that he could turn an Eldian captive into a three- or four-meter Titan, and then successfully did it.
Marley's Public Security use the injection as a life sentence against Eldians who break the law, including the Eldian Restorationists, to roam Paradis Island as Pure Titans for the rest of eternity as they eat other Eldians.
The Scout Regiment have used it as a means to save the lethally wounded by turning that person into a Titan, then having it devour another with the power of the Titans to restore them to human form.
Around 60 years before the fall of Wall Maria, Ymir and her cult of Ymir-worshipers were discovered by the Marley Public Security, 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 sent to Paradis Island. Despite Ymir's pleas for mercy, she and the other cultists were transformed into Titans by the use of an injection.
In the year 832, a rebellious group of Eldians in Marley, the Eldian Restorationists, was discovered by the Marley Public Security, thanks to Zeke Jaeger. They were rounded up, sent to Paradis Island, and were turned into Pure Titans; however, one of the Marleyan officers was revealed to be Eren Kruger, "the Owl," and saved Grisha Jaeger from this fate. After giving Grisha a mission to reclaim the Founding Titan, Kruger injected him with the serum so Grisha can eat him and inherit the power of the Attack Titan and reach the Walls.
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, Grisha Jaeger traveled to the Reiss Chapel to speak with Queen Frieda. After battling with Frieda in her Titan form and slaughtering most of the Reiss family, he reunited with his son, Eren, and injected him with a serum, transforming him 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.
Five years later 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 Titan. In order to save his comrades from the collapse of the Chapel's caverns, Eren bites into an injection vial labeled "Armor," granting him the ability to harden his 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.
Dhalis Zachary meets with Dot Pyxis, Erwin Smith, and two of his Scouts to discuss the syringe left behind by Kenny Ackermann. Hange Zoë explains that its contents seem to originate from some sort of human bodily fluid, but it evaporates when exposed to open air, making it difficult to study. Hange suspects that whoever made this had access to a higher level of technology than they do. Since they cannot tamper it without ruining it, Pyxis suggests they use it for their current objective, and the syringe is entrusted to Levi Ackermann.
The injection given to Levi later becomes a subject of conflict as both Armin Arlelt and Erwin Smith are brought to him on the brink of death. Ultimately, Levi chooses to save Armin with the injection, and Armin's Pure Titan consumes Bertholdt Hoover, the Colossal Titan, and Armin inherits the Power of the Titans.
In 854, the Anti-Marleyan Volunteers present a large stock of Titan serum to the Garrison, explaining they had stolen it from Marley. However, Yelena admits duplicating it will be hard since they were unable to obtain any of the materials, equipment, or experts in Titan science they need. Dot Pyxis thanks the volunteers, while he begrudgingly arrests them over uncertainties regarding their leader.