Med-X

Med-X is a the name of the highly potent painkiller morphine which appears in Fallout 3. If one happens to find the Prototype Medic Power Armor in the sewers of Old Olney, Med-X will be automatically dipensed every 2.5 hours (game time) or so after checking your hitpoint total. (Note that Med-X will only be administered when your health total drops below a given amount, and THEN will it be administered every 2.5 hours. So the first dose will normally occur in battle, when you need it.). While being administered this way, you cannot develop an addiction.

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.