RadAway is an intravenous chemical solution that bonds with radioactive particles and removes them from the user's system. While Rad-X is designed to increase the body's natural resistance to radiation, RadAway is designed to be used after exposure. Purging the body of radiation takes some time to complete, and anyone using it will experience its adverse effects: it's a potent diuretic and can cause nausea, diarrhea, stomach pains and headaches.
In Fallout, Fallout 2 and Fallout Tactics, interestingly RadAway can be addictive. It takes full effect only after four in-game hours. RadAway is also one of these games' higher-value items.
↑Fallout Official Survival Guide p.124: "RadAway: Used after you have taken radiation damage, this drug will remove some of the contamination from your system. It takes time for the effects to work. There is a chance of developing an addiction to this drug."
↑Vault Dweller's Survival Guide p.5-18: "If you have been exposed to large amounts of radiation, then use RadAway to remove the radiation from your system. It won’t feel good, but better a headache and some stomach problems for a couple of days than the long term effects of nuclear radiation! RadAway takes a little while to work."