Instant Sedation basically doesn't exist in Real Life without a substantial risk of killing the recipient. It can take anywhere from several minutes to several hours for any given sedative to take effect. But since television only gives you 40-odd minutes to work with, this is never fast enough — and yet TV sedation is almost always easily survivable. The only reason to make the sedation time longer is to play it for laughs. There are several ways to administer a sedative:
- Injection or Tranquilizer Dart. It's important to distinguish between intravenous injection (i.e. directly into the bloodstream) and intramuscular injection (i.e. wherever the heck you can jam a needle). Intravenous injection of some anesthetics can indeed knock out a person in under 30 seconds, but on TV, your typical sedation target is probably going to resist vigorously, so you can't exactly aim for a vein. Intramuscular injection can take upwards of 15 minutes to take effect; it's evident in safari shows when they shoot an animal but keep trailing it for several minutes waiting for the drug to take effect. Sometimes, to show how tough a character is, he might be shot with multiple needles or darts; this will indeed make the sedative work faster, but again also increases the risk of fatal overdose.
Some works might be aware of this and have a character like a doctor stab the victim in precisely the correct spot to give an intravenous injection, which in Real Life can only work with blind luck. Also, some drugs might work intravenously but not intramuscularly, so if you miss, you're just going to cause bad bruising at the minimum. And fiction tends to disregard the fact that there are some incredibly stupid places to try and stab someone with a needle. Most Real Life tranquilizer darts are aimed at the rear end rather than the neck, as it's much easier to hit.
If you miss and hit an artery, that causes all sorts of issues, namely: (1) the drug has to travel down the capillary network before being picked up by the venous network, which not only takes longer but will also damage the tissues around the capillaries; (2) arteries tend to be much thicker than veins and thus harder to penetrate with a syringe; and (3) it will bleed a lot. In medicine, there are only a few very rare circumstances in which any drug will be injected into an artery, and they require special equipment to prevent the body from shoving blood backwards up the needle. Veins are your friend.
- Chloroform rags are popular in things like Hardboiled Detective novels where the goons want to kidnap somebody by shoving a chemical in their face. On TV, the victim collapses instantly. In Real Life, it takes several minutes to render someone unconscious this way, and it does occasionally kill the intended victim, so Don't Try This at Home. That said, administering a drug by inhalation is only marginally slower than by injection (which still doesn't help very much).
- Knockout Gas is a common way to knock out many people at once, especially in works which can't show dart guns or hypodermic needles. And it's frequently visible to the audience in a nasty color to show that it's working. The gasser may occasionally go to the effort of putting a mask over his own face; sometimes he gets away with just covering his nose. This is because not only does the gas work instantly, but it also happens to be very selective in whom it affects.
In real life, such gases do exist, but they take a lot longer, and they do run the risk of killing innocents. Dosage is important; it generally takes more to knock out a big tough guy than an Innocent Bystander (particularly women and children, who tend to have lower body mass). Works remember this if they want to show how tough a guy is; they forget this when Knockout Gas is used and the first people to be affected are the innocents rather than the rather large and intimidating bad guys. The police in Moscow found this out the hard way when they used knockout gas in the 2002 Moscow theatre siege disaster. Also, since the gas is much less concentrated in the air than it would be on a chloroform rag or an anesthesia machine, it tends to be much less efficient than these methods.
- Paralytics are not the same as sedatives; a big enough dose of a paralytic drug or gas can actually work kind of similarly to how fiction thinks tranquilizers work. The catch is that paralytics stop all voluntary muscles, including the ones that control breathing. In medicine, they're never used without a ventilator or similar breathing equipment, so using them as Instant Sedation will likely kill the victim.