A point would be reached where beneficial effects would disappear and harmful effects would start to be noticed. Drinking too much water causes water intoxication. In severe cases, this kind of poisoning causes convulsions and seizures. There are reports of such poisoning in small children and in psychiatric patients. This relation is true for all chemicals regardless of whether they are natural or man- made.
Toxicity is a measure of the poisoning strength of a chemical.
Chemicals that are only weakly toxic require large doses to cause poisoning. Strongly toxic chemicals only need small doses to cause poisoning. Toxicologists often use animal tests to determine whether small or large doses of a particular chemical cause toxicity. There is a tendency to think of chemicals in terms of those which are poisonous or toxic and those which are harmless. This is not the case because any chemical can cause poisoning if a sufficient dose of it is taken into the body.