RadAway is an intravenous chemical solution that bonds with radiation particles and passes them through the body's system. It takes some time to work, and is also a potent diuretic.
It cannot be used to keep away from, or resist radiation when entering radioactive areas like its relative namesake Rad-X, but is instead used after receiving radiation to purge some of it from the body.
On top of being addictive, it has several adverse effects, most notably headaches, stomach pains, and hair loss.
RadAway is addictive in Fallout, Fallout 2, and Fallout Tactics. RadAway is not addictive in Fallout 3, Fallout New Vegas, Fallout 4 and Fallout 76.
In Fallout 3, Fallout: New Vegas and Fallout 4, RadAway has the appearance of an I.V. bag with "radaway" written on it in marker pen. However, in Fallout 76 it comes in I.V. bags branded with the RadAway logo.