Heart attacks are easy. Inject an air bubble into a vein.It can be done with a dart that then falls away and leaves only a teeny little red freckle. In the USSR it was known as the Kremlin Flu.
It takes more than a couple of bubbles to kill you and a dart to his a vein... too many movies. cmon man!
A nurse is standing right by you, and still she needs your assistance to hit the most convenient vein with a needle. Veins are not large, and they are not easily visible. A successful injection of anything with a dart into a vein, through clothes, from a distance, is unrealistic as a weapon. An agent who operates a covert dart blower would be lucky to hit the target at all.