Med-X is a potent opiate analgesic that binds to opioid receptors in the brain and central nervous system, reducing the perception of pain as well as the emotional response to pain. In layman's terms, it's a run-of-the-mill pain killer delivered with a hypodermic needle. According to Boxcars, 15 hits of Med-X is more than enough to kill a man.
Med-X is a potent pain killer that can often be found in first aid boxes or abandoned medical facilities. The chem will increase your damage resistance by 25%, but there's also a 10% addiction rate that results in a -1 penalty to Agility and Intelligence.
Med-X was originally to be named Morphine in Fallout 3, but this resulted in the Australian Classification Board refusing its classification and thereby making it illegal to distribute the game in Australia. The game was later censored to remove all real-world drug references including changing morphine to Med-X in order for the game to be allowed in Australia. This censorship was to become part of the worldwide release.