Rad-X (Fallout 3)

Rad-X is an anti-radiation chem to be taken before exposure (unlike Rad Away, which removes the effects of radiation after a person gets irradiated). It has no known side effects.

Fallout, Fallout 2 and Fallout Tactics
In Fallout and Fallout 2, Rad-X usage was capped: taking a double dose doubled the effect too, but taking a third one didn't do anything. It wasn't really necessary, since in Fallout, taking a double dose grants 100% Radiation Resistance (enough to survive the most irradiated place on the map for several hours) and in Fallout 2, significant radiation can only be obtained in the Toxic Waste Dump random encounter, the Gecko reactor core and getting hit by Aliens, all being avoidable.

Fallout 3
Rad-X is an anti-radiation chemical that can significantly reduce the danger of irradiated areas. When combined with an Advanced Radiation Suit, Rad-X can—even at moderate medical skill levels—create near-immunity to radiation.

In general, Rad-X is best used in heavily irradiated areas, as that is where the most benefit can be seen. It is reasonably common and, as such, it can be frequently used as the situation warrants. Plus, like Stimpacks and Rad-Away, you cannot develop an addiction to Rad-X. However, taking multiple doses of Rad-X is pointless as it does not increase your resistance to radiation any further. (In other words, the effects do not stack with multiple uses or Rad-X)

Appearances
Rad-X appears in all Fallout games.