Potasium iodide tablets (for radiation poisoning)
Potasium Iodide only works fo cesium and other stuff from a nuke explosion, not a dirty bomb.
Dirty Bomb is just uranium or other stuff, I doubt we will see a plutonium bomb
So, aside from doing absolutely nothing helpful for you in the case of anything other than radioiodine, there is a slight chance of an adverse allergic reaction, just as there is for any dietary supplement. I think the statistics are an average of 1 person in 2 million, or maybe its two people in a million, but you get the general sense from those numbers. Thus, for the case of a large population that panics and starts gulping down KI tablets because they heard "there's a 'dirty bomb' going off", probably the reaction to the KI is going to present more of a hazard.
Potasium iodide tablets will only protect you from getting thyroid cancer. It will not protect you from any other cancer nor will it protect you from radiation poisoning itself.
Also I always keep antibiotics. Radiation damages the immune system.
Actually that won't protect you very much. A San Diego company, HollisEden Pharma, is developing a radio-protectant. They are ready to start Phase III monkey tests and Phase I human safety trials. The drug is being developed to counter the effects of radiation from a dirty bomb or nuclear detonation.