Cyanide is a lethal chemical that was commonly used in suicidal pills throughout World War II to prevent captured spies and high-government officials from leaking their country's secrets to the enemy. Although the cyanide pills were deemed a 'quick and painless' death at the time, modern science now shows us that this wasn't the case; instead, those who took them probably experienced a long and agonizing death.
Cyanide works by blocking a cell's ability to produce ATP, a source of energy used by your body to perform natural functions. When a person ingests a lethal dose of cyanide, the body's cells suddenly stop producing ATP. After this occurs, muscle cells don't get the energy they need, and the person experiences inconsistencies with muscle contraction and relaxation.
Your lungs' diaphragm and your heart are pure muscle; when they can no longer function, you won't be able to breathe, and cardiac arrest will ensue. The length of time it takes for cyanide to kill a person ranges from around 2-5 minutes, and during this timeframe, they're fully conscious and experience every bit of the chemical's effects.
Cyanide acts fast because its molecules are tiny, allowing it to spread through the body rapidly and efficiently. On the other hand, a conscious death lasting 2-5 minutes will feel like an eternity for the affected person, so cyanide poisoning should probably be one of the last things on the world's list of "preferred ways of dying."