A hypodermic needle is used to inject various poisons to kill or incapacitate an enemy. Such needles can be disguised for field use and can be filled with a variety of poisons from poisocain, fish paralyzer, or tetrodotoxin
Archer gives Cyril a hypodermic needle disguised as a pen full of what he claims is a highly toxic poison called Poisocaine, in "Training Day." The pen cap is loose and slips off for no reason. It fell off in Cyril's pocket which injected Trinette when she hits him. It is unknown if the pen was loaded with poison, or if it was just a tranquilizer as Archer claims.
During "A Going Concern," Cyril uses a hypodermic needle full of fish paralyzer on Len Trexler as part of Sterling's plan to force Len to fall out of love with his mother and prevent her from selling ISIS.