You're out wandering the wasteland when suddenly your Geiger counter stars going berserk. Oh no, you've been irradiated! No worries, the Department of Defense has you covered. Just pop some Ex-Rad pills and you'll be fine.
There have been decades of research on ways to protect humans from radiation, like lead and concrete shielding or radiation suits. There hasn't been much you could do once you're exposed to radiation, though. Potassium iodide can protect you from one specific type of cancer (thyroid cancer) for one specific type of radiation exposure, but otherwise it's just, "evacuate the area, and quickly."
Onconova Therapeutics, with assistance from the DoD, have developed a new drug called Ex-Rad that increases the likelihood that you'll survive if you are exposed to a potentially lethal dose of radiation. You can take it before or after exposure, intravenously or via pill. Studies with mice have shown that mice who got a the drug and were exposed to a lethal dose of gamma radiation had a higher survival rate than mice who did not receive Ex-Rad. Human tests did not use radiation, but showed that the drug was not harmful and had no side effects.
Researchers also conducted experiments with cell cultures to try and figure out how Ex-Rad works. It seems to alter how cells deal with DNA damage and suppresses a specific protein (p53) that triggers cell death.
This is one of those "science-fiction predicted it" stories thanks to the Fallout video game franchise. Fallout players have been gulping pills for years to reduce their level of radioactivity and protect against high radiation levels. The drug that protects you from radiation is Fallout is even named Rad-X. I wonder if the DoD researchers are gamers.
Sources:Ghosh, S.P. et al. "Radiation protection by a new chemical entity, Ex-Rad: efficacy and mechanisms." National Center for Biotechnology Information.
Onconova. "Ex-RAD® for Protection from Radiation Injury." Onconova.