You can only use this cheat after you have talked to (and fully extended the conversation) everybody in the underworld concourse which is at the Museum of History next to the Washington Monument.

Go find Moira Brown, she should be wearing a RobCo sweatsuit.

When she's done taking, ask her about her survival guide book that she writing about (if she didn't bring it up already).

After you asked her about it, then chose the dialogue sentence, "Can I help with the first chapter, or something like that.

After you do that, ask her about healing really bad radiation sickness.

When she says this might hurt a little, go with it.

Your screen will go black for about 5 seconds and you will have zero radiation sickness at all.