Why is cyanide lethal? What dose it do once inside the body?

it is like a tranquilizer, once in the blood or respiratory system, it is capable or damaging the cells surrounding it and causing your immune system to kill those cells and the healthy ones around it.

one it reaches the brain it could cause internal brain damage from inside the brain instead of outside. it will shrink the cells or may implode then because of the lack of oxygen and neural transition form the brain to the muscles causing paralysis or even death

In large doses it stops cells using oxygen causing these cells to die. So in a small amount it would not kill you. So lets say cyanide is inside the body it would kill off all the cells and your body would not be able to function causing death