The substance in the darts causes people to obey whatever the person who shot the dart orders. The victim of this poisoning experiences extreme disorientation and clouding of their vision, and they see a strange light towards which they will go and carry out the shooter's orders. The victim can also be very hostile towards their friends and run away from or attack them.
The Dart Gun seems to be an important part of how Marlowe's secret society uses fear to control people. This neurologic agent is a part of the chemical arsenal that Marlowe and Talbot are attempting to augment by acquiring the hallucinogen from Iram of the Pillars.
The neurologic agent in the dart gun is not named.
The dart gun is used twice during the course of the game. One time is in Syria, when Talbot uses it to get Charlie Cutter to give him a notebook and attack Nathan Drake. The second time is in Yemen when Talbot shoots Drake with it to make Drake leave his friends and go to Marlowe. The dart gun itself is never clearly seen; In Syria there is a brief shot of Talbot putting it away after shooting Cutter, and all other times only the dart itself is visible. It appears to use the Tranquilizer Dart Gun model from the previous game.