If you are a human being, then you're probably aware of "knock-out" darts: small, painless darts that knock out a human in a few seconds after being shot with no serious after-effects.
Like the knock-out punch, which has similar literary and film uses, (I'm looking at you, Lost!) the knock-out dart doesn't exist. Although it is possible to knock out a human with a dart, you cannot have that injection be both fast and safe. If you want to knock out a human in seconds, his heart will stop in minutes. Soldiers would kill for a dart that worked like the one in the media (and they've often killed for lack of it), but no amount of funding has produced this miracle.
PS. I saw Ratatouille again. Excellent.