There’s no real “safe, reliable way” to knock someone out that’s not in controlled circumstances. A blood choke like the triangle choke where the elbow wraps around the neck to cut off blood supply to the brain will knock someone out very quickly, but it will also kill them. It happens very quickly, so the margin for error ends up being a matter of seconds. In the heat of the moment, too much relies on aggressor’s discretion and their enemy’s physiology. The brain needs blood to function, if the flow of blood suddenly stops then the brain can no longer work and it shuts down. This is what causes the knock out, but knocking someone out is basically putting them in a coma and one step away from death.
The same is true of oxygen deprivation. When you choke someone, you’re strangling them. You’re cutting off oxygen to the brain by obstructing their ability to breathe. Many chokes apply direct pressure to windpipe by squeezing with the hands or crushing with the forearm. The techniques always risk permanent damage to the windpipe and to the brain.
In a controlled environment like a martial arts match or a UFC bout where there are referees keeping careful watch on the contestants and are ready to leap in at a moment’s notice if something goes wrong (and the contestants are given the option to tap out before they pass out), this isn’t as much of an issue. The same is true of the Army and the Marines who both teach choke holds, including a more deadly variation on the triangle choke, because they are effective techniques in situations where the survival of the enemy isn’t an issue. Police in the United States used to love choke holds because they are very effective, the reason they aren’t used anymore is because policemen who used the techniques accidentally killed a great many suspects while subduing them. (The same is becoming true of Tazers. Yes, freaking out someone’s nervous system with electricity can in fact kill them.)
Drugs, in the controlled environment of a hospital they work very well, in a combat situation where you can’t control all the variables not so much. There’s also considerations like body weight, height, and resistances to various drugs that vary from person to person. Unfortunately, there are no one size fits all drug types and in a combat situation too many things can go wrong for it to be reliable. Add to that, any time you put someone under there’s a chance they won’t wake back up or will wake up with real brain damage and it just isn’t a viable solution.
This is all before we get to the issues of moving the body. Moving an unconscious person is a lot like moving a corpse (except they could wake up and, while you can guesstimate, you don’t know for certain when that will be). While putting someone over your shoulder in a fireman’s carry works, it’s incredibly aggravating and terribly obvious. Dragging the body is slow and cumbersome, while carrying it with two is awkward. If you saw a pair of guys in black dragging an unconscious body into the back of a black van, you’d probably call the police. On the other hand,if you saw a nicely dressed man putting an obviously drunk twenty something into the back of a taxi cab, you might not question it as quickly.
Whether it’s a hold up on heroes sneaking around a government facility or a snatch and grab off the street, it’s much more viable to make your target move themselves. Kidnappers don’t have fifteen to twenty minutes to exit a scene, they have five. They’re moving fast. This means disorienting their target and using the fear, shock, and trauma of being kidnapped to force them to move. Whether it’s getting hit with the butt of the rifle, a black bag and handcuffs, or getting dosed with an animal tranquilizer (or date rape drug like rohypnol), it’s much more viable to put them in a condition where they can’t struggle or fight back and make the target carry themselves.
I do understand the dilemma here. We’ve been conditioned by countless action movies to believe that a knockout is an easy out. The enemy cracks the character over the back of the head and we change scene. Unfortunately, (and if you look at most movies that deal with realistic kidnapping like Man on Fire, you’ll notice a change in tone) this isn’t how it actually works. The goal of a kidnapping is going to be extracting them alive and scared, but relatively undamaged. You’re taking them because they are valuable to someone (whether it’s for ransom or for sale is less relevant). Anything that jeopardizes that ultimate goal is going to be off the table for a professional. If you’re writing an amateur kidnapper, they may go for a knockout because “that’s how it works in the movies”.
In Hollywood and some books, knockouts have become sort of a “free pass” for badasses. The badass gets to do all the fancy tricks and cool moves but can also get the “good person cred” of not killing anyone. It’s a “have your cake and eat it too” bit where the author handwaves the violence and doesn’t deal with the consequences. It’s in the same range of heroes shooting arrows and bullets through joints and going “Ha! See! I’m super skilled and I don’t have to kill!” but avoids the obvious part about CRIPPLING THEM FOR LIFE!
This isn’t to say you can’t go with it but just remember, no matter what your hero does, if they are using violence then they are always running the risk of killing someone. This is especially true when harming vital organs or the brain.