Basic Expert: Too much of a good thing is... bad?

Apparently too much of anything is bad for you. In fact at a certain point it has 50% chance of killing you.

Poisonous Substance                       DC 10
Water (H2O)                                     90g/kg body weight or
                                                            1cn/lb body weight.

Sodium chloride (table salt)          3g/kg body weight
                                                            1cn/30lb body weight

Ex. Arlo the Torturer pours waterskin (5cn empty/ 30cn full=25cn water) after waterskin of water down your 200lb PC's throat at a 2.5lb of water per waterskin. After eight waterskins full of water your PC requires a DC10 check or he dies.

So how does neutralize poison work if the substance is not poisonous unless the dosage is fatal? Lets say your PC must survive a DC 10 poison check before you can use neutralize poison.