The Anti-Radiation Pill is a medical item which will lower your Radiation by 200. Anti-Radiation Pills are mostly being used to be able to enter areas which are contaminated with radiation so players are actually able to loot crates inside buildings. If the player does not use the pills he will start losing health as soon as his "Rads" value reaches 500.
This item cannot be researched. You must find this yourself at compounds or in the loot bags dropped by red wolves and bears when they die, while they are not a common find.