Before the game starts, for every point you choose to put into constitution you get 20 bonus HP, however it no longer applies for later increases, either from equipment, food or perks. I know it may take a lot to code, but it might make the constitution stat a lot more useful if the HP increase also happens after character creation, eg the gas mask can also give a 20 HP bonus since it increases the constitution stat by 1 while nature's anguish would be removing 50 HP since it removes 30 HP and 1 constitution. (If anyone thinks that keensense and intelligence don't get much use, constitution only has two checks (drinking in nogas and one of the quest for greyfold) and they're optional)