Drug users inject themselves every day, even when they're high. If they can do it, anyone can do it. The thing to consider is what to inject... some people think that injecting bleach or something would do it (and some people think injecting an air bubble will do it), but that's not true... that's more likely to hurt really bad and have some nasty long-term effects. It would take a good bit of research into the type of injection and the amount... for example, a large dose (OD) of H would absolutely do it, and causes a lot of druggies to unintentionally CTB every day, but a slightly smaller dose would just knock you out for a while and possibly lead to addiction.
Personally, I find injections too uncertain to try them, but they can absolutely be effective.