BBC News Online looks at the effects and use in warfare of substances such as hydrogen cyanide, which can kill in minutes.
Very little cyanide is needed to kill
Main agent: Hydrogen cyanide.
Effects: Hydrogen cyanide is absorbed through the lungs into the blood. It kills body tissues by destroying their ability to take in oxygen, particularly affecting the brain and heart. The heart can stop within six to eight minutes of exposure. Hydrogen cyanide can also be absorbed through the skin.
Recognition: Colourless gas, which may smell of bitter almonds or peach kernels.
Use as a weapon: Needs to be used in confined spaces rather than the battlefield to be effective.
History: Produced by France during World War I and used by the Nazis in the form of Zyklon B in their gas chambers during World War II.
Production: Hydrogen cyanide is used in the worldwide manufacture of acrylic polymers, so there is some potential for it to be acquired by those wishing to use it as a weapon.
Protection: Gas masks protect against hydrogen cyanide. There are some antidotes, but their effectiveness is debatable.