Med-X is the brand name of morphine, 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. The active ingredient acts as a general anesthetic that dulls pain throughout the body and typically causes listelessness and lethargy. In layman's terms, it's a powerful pain killer delivered via a hypodermic needle.
Given its usefulness, Med-X is one of the more widespread pre-War chems, providing pain relief and recreation across the continent. Morphinists can be identified by contracted pupils and involuntary spasms.
↑The Courier: "Contracted pupils, involuntary spasms... Med-X kills quickly, Joana. I can help." Joana: "[SUCCEEDED] I... I didn't think it showed.I guess that it's worse than I thought. I... look, we can't talk here. If you think you can help me, follow me to my room, please." (Joana's dialogue)