RadAway is an intravenous chemical solution that bonds with radiation particles and passes them through your system. It takes time to work, and is also a potent diuretic. It cannot be used to keep away radiation when entering radioactive areas like Rad-X, but instead is used after receiving radiation to rid yourself of some of it.
It has several adverse effects, most notably headaches, stomach pains, and hair loss.
RadAway is also one of these games' higher-value items, and since you'll find far more than you'll ever realistically use, it is very good for making money.