Serum was an anti-venom used by Naked Snake, during Operation Snake Eater, to cure poisoning inflicted by the wildlife native to Tselinoyarsk. If left unchecked, poisoning would have resulted in his health slowly diminishing until death.
Snake's serum was useful for treating toxins from a variety of poisonous animals, such as the Cobalt Blue Tarantula, the Emperor Scorpion and the Brazilian Wandering Spider (the last of which he never encounters directly, rather it was used by The Fear for his darts). He was forced to use it when fighting The Fear, as he poisoned Snake by firing venom-coated arrows with his crossbow derived from the Brazilian Wandering Spider. The rabbits inhabiting Graniny Gorki also developed an immunity to this venom via a blood serum, due to The Fear having previously tested his arrows on them.
|Warning: The following information is from outside Hideo Kojima's core "Metal Gear Saga." It has some level of canonicity within the continuity, but reader discretion is advised.[?]|
|Non-"Metal Gear Saga" information ends here.|
During the Peace Walker incident, both the Peace Sentinel CIA mercenary group and the Soviet soldiers at the United States Missile Base in Nicaragua utilized syringes similar to the serum in the event that they were wounded, also waving the syringe against various Militaires Sans Frontières members if they came close to ward them away.
Behind the scenes
The Emperor Scorpion's venom is not actually lethal in real life.