I mean, they're all nonlinear at some point in the range of possible doses. Radiation too. I should have specified I was focusing on the "very low" bit of the curves. Chest X-ray vs two chest X-rays, rather than Louis Slotin vs the guy at the back of the room.
Nope. Most medicines are totally nonlinear. Almost any function is affine if you look close enough, but that's different from linear. If you half the dosage of ibuprofen, your odds of complications rarely fall by 50%.
Chest x-ray is pretty linear within reasonable dosage ranges, for much the same reason I'd expect mRNA damage to be.
