why do we die when we eat or drink poison?

Poisons are substances that can cause damage, illness, or death to organisms, usually by chemical reaction or other activity on the molecular scale, when a sufficient quantity is absorbed by an organism. Different poisons harm the body in different ways.

A poison obstructs or inhibits a reaction, for example by binding to a catalyst. Contact or absorption of poisons can cause rapid death or impairment.  Agents that act on the nervous system can paralyze in seconds or less.

Inhaled or ingested cyanide almost instantly starves the body of energy by inhibiting the enzymes in mitochondria that make ATP. Intravenous injection of an unnaturally high concentration of potassium chloride, quickly stops the heart by eliminating the cell potential necessary for muscle contraction.