The syringe gun is a piece of equipment cut from Counter-Strike: Condition Zero Deleted Scenes.
The only references to it are the view model, world model, HUD icons, and console error messages.
Upon starting Condition Zero: Deleted Scenes, the console displays these messages:
"Warning: Unable to open sound/weapons/syringe_use.wav for transfer
Warning: Unable to open sound/items/mapdown.wav for transfer
Warning: Unable to open sound/items/mapup.wav for transfer
Error: server failed to transmit file 'sound/weapons/syringe_use.wav'
Error: server failed to transmit file 'sound/items/mapdown.wav'
Error: server failed to transmit file 'sound/items/mapup"
The "Warning: Unable to open sound/weapons/syringe_use.wav for transfer" and "Error: server failed to transmit file 'sound/weapons/syringe_use.wav'" error messages indicate that the syringe gun was originally going to be in the game with at least one unique sound. Even though it was scrapped and no syringe gun sound files are included with the game, the coding was never removed for an unknown reason.
It is unknown what (if anything) liquid this item was intended to administer, let alone, what its intended target was.
Unused quotes from the hostages suggest that wounded hostages would be either slowed or immobilized, requiring first aid before moving any further. Said quotes thank the player for giving them a "shot". This suggests that either the Syringe gun or the Portable Medkit were to be used on hostages. If the former was true, than the Syringe gun may be originally intended be used to heal the injured hostages. It's unknown if the player can heal their teammates with it.
Note: The weapon may have been intended for other things than administering liquid, and it's purpose is unknown. This article is mostly conjecture.