RadAway

Rad Away is a chemical solution that bonds with radiation particles and passes them through your system. Takes time to work. It is also a potent diarrhetic. While Rad Away removes the negative effects of radiation, it does not prevent you from becoming irradiated, unlike Rad-X.

Fallout, Fallout 2 and Fallout Tactics
(More about Rad Away in Fallout 3 below)

Fallout 3
In Fallout 3, Rad Away appears to no longer be addictive. However, it may be that it merely has a much smaller addiction rate. Whether this is due to game redesign or if in fact this is a different variant due to locale differences is undetermined.

See also:
 * Fallout 3 drugs and edibles

Rad Away website
Rad Away appears to have been featured by a real Rad Away website

Appearances
Rad Away appears in all Fallout games.

Anty Rad