Warp injector casings were a component of warp injectors.

Presumably, the injector casings either held, or were an actual part of warp injectors. That much however, is not made clear in "The Seventh"

In the mid-22nd century, transporting spent casings was a profession that was considered an unpopular job, but a necessary one. It was noted that one would not "find too many old men hauling spent injector casings" as it would elevate hemolytic cell count, and eventually cause death.

Menos had been using transportation of spent injector casings as cover to smuggle biotoxins, and by 2152, would have an elevated hemolytic cell count over 3,000. He would claim that this would eventually kill him, effectively taking him away from his family. (ENT: "The Seventh")