|You're doing it wrong.|
The latest trope seems to be that the injector sneaks up behind the injectee, randomly sticks a foot-long needle into the injectee's neck, slams the plunger down, and then watches the injectee slide senseless to the floor.
Not only is this trope now way overused, it's also horribly unrealistic.
It probably started with some writer or director thinking it would be so "hard core" to have a drug shot right into the carotid artery so it would hit the brain right instantly. Then everyone saw that and started using it themselves. Peer pressure, ya know.
If you think a moment about the anatomy of the human neck, there's a hell of a lot of stuff going on in a very small space. Major arteries, veins, tendons, nerves, vertebrae, trachea, muscles, and probably a bazillion other things all packed together. So if you're going to jab a needle in there (or use a Star Trek hypo-spray for that matter), from behind, while the patient is struggling, just how well do you think you're going to do at hitting a 5mm carotid artery?
Let's say that you hit the jugular vein instead. The drug would then need to go through the heart before getting to the brain, just like it would if it were injected into any other vein in the body.
But you might also hit the trachea instead (which seems more likely since it's about 250mm in diameter). Either the needle is going to bounce off that and send the drug into a muscle or who knows where, or it will go into the trachea and send the drug into the lungs.
All of this is completely ignoring the fact that certain drugs must be administered differently. Some need to be injected into muscle. Others need to go into the layer of fat under the skin. For either of those the neck is a terrible place to inject them (for intra-muscular injections the best site is a large muscle like the thigh or shoulder, and for sub-cutaneous the preferred site is the belly).
Yet in recent movies and TV programs I keep seeing scenes where a protagonist needs to be stopped by an ally by being being injected in the neck (presumably in the carotid artery) with an anesthetic, even though that would likely cause death or a stroke.
Look guys, let's just go back to the spray injection into the shoulder or the tranquilizer dart to the butt. That's still far-fetched (it usually takes a lot longer for that stuff to work) but it's not quite as silly and doesn't squick me out.