There are no injections in Lois Lowry's The Giver. The morning injections, however, are shown in the movie adaptation. The book has everyone take pills each morning, but the movie does show Jonas swiping his wrist over a technological device to receive the injections rather than taking a pill. The injections in the movie offer the same treatment as the pills in the book, though. Both are first administered to young adults as they start to experience the "stirrings." The stirrings are the beginnings of sexual awareness, basically. As each person takes the pill each day, these feelings are suppressed so no one will prefer one person over another. In a world where Sameness is the doctrine for living, if someone preferred to be with one person rather than another, then feelings of differentiation would start to exist--and they can't have that in a society that prohibits living with one's own birth parents.
"Pedaling rapidly down the path, Jonas felt oddly proud to have joined those who took the pills. For a moment, though, he remembered the dream again. The dream had felt pleasurable. . . he thought that he had liked the feelings that his mother had called Stirrings. . . Then, in the same way that his dwelling slipped away behind him as he rounded a corner on this bicycle, the dream slipped away from his thoughts" (39).
As shown above, the pills work very quickly to eliminate the thoughts as well as the feelings that are associated with anything sexual. Jonas isn't down the street for more than a minute before he forgets about the dream he had and the Stirrings go away. This is just another way that the community controls people from a very early age in order to ensure their safety and status quo.