Hydra is a drug developed from antivenom. Due to the Legion's disapproval of using modern medicine, some Legionaries attempted to develop a different means to help them heal damaged limbs. To do that, they combined cave fungus, nightstalker blood and the poison from a radscorpion poison gland with antivenom before use. This resulted in the development of Hydra, a curative agent that both anesthetizes and restores crippled limbs over time.
Despite its origins in the Legion, this drug has since seen extensive use throughout the entire Mojave Wasteland, including NCR field hospitals. Excessive use causes dilated eyes, an increased heart rate and blood pressure, excessive sweating and other typical symptoms of drug abuse.[1][2]