Rad-X increases resistance to radiation by 50% per dose, maxing out at 100%. Further doses have no effect. Rad-X has no practical use and can be safely sold.
↑Vault Dweller's Survival Guide p.5-18: "Rad-X is a preventive medication. Take Rad-X before exposure to radiation, and the total amount of radiation you receive will be reduced. Rad-X bolsters your bodies own Radiation Resistance. The stronger you are to start with, the more effective Rad-X will be."
↑Fallout Official Survival Guide p.124: "Rad-X: A radiation prophylactic. Use this drug before you enter a radioactive area, and the total amount of radiation you receive will be reduced. Rad-X increases your body's natural Radiation Resistance."