Med-X

Med-X is the name for the highly potent painkiller which appears in Fallout 3. When injected, it makes the user impervious to pain to the degree that damage resistance is increased by 25% - this will last for 240 seconds, or twice as long with the Chemist perk.

Applications in Prototype Medic Power Armor
Besides providing the benefits of regular power armor, the Prototype Medic Power Armor, found in the sewers of Old Olney, will automatically dispense Med-X every 2.5 hours (game time) after checking your hitpoint total. While being administered this way, an addiction to Med-X will not develop.

Behind the scenes
Originally, Med-X was simply named "morphine" in the game. However, on July 4, 2008, Fallout 3 was refused classification by the OFLC in Australia, thus making the game illegal for sale in the country due to the "realistic visual representations of drugs and their delivery method (bringing) the 'science-fiction' drugs in line with 'real-world' drugs." On September 9, 2008 Bethesda vice president of PR and marketing, Peter Hines, has described the idea of the Australian version of Fallout 3 as a misconception. All versions of Fallout 3 will no longer include real world drug references. However, the name "morphine" is still used in the Japanese version of the game.