Sarin, a deadly nerve gas which the United States now says the Syrian regime has used against rebel forces, was developed by Nazi scientists in 1938.
Originally conceived as a pesticide, sarin was used by Iraqi dictator Saddam Hussein's regime to gas thousands of Kurds in the northern town of Halabja in 1988.
A cult also used the odorless, paralyzing agent in two attacks in Japan in the 1990s.
White House officials said Thursday that US intelligence agencies, working with European allies, concluded that Syrian President Bashar al-Assad's regime had employed the gas in its fight with rebels.
France and Britain have already said lab tests of samples from Syria showed the regime had resorted to chemical weapons.
Inhaled or absorbed through the skin, the gas kills by crippling the respiratory center of the central nervous system and paralyzes the muscles around the lungs.
The combination results in death by suffocation, and sarin can also be used to contaminate food or water supplies, according to the US Centers for Disease Control and Prevention (CDC), which notes that antidotes exist.
"Sarin is 26 times more deadly than cyanide gas. Just a pinprick-sized droplet will kill a human," according to the World Health Organisation.
Exposure symptoms include nausea and violent headaches, blurred vision, drooling, muscle convulsions, respiratory arrest and loss of consciousness, the CDC says.
Nerve agents are generally quick-acting and require only simple chemical techniques and inexpensive, readily available ingredients to manufacture.
Inhalation of a high dose -- say 200 milligrams of sarin -- may cause death "within a couple of minutes," with no time even for symptoms to develop, according to the Organization for the Prohibition of Chemical Weapons.
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Exposure through the skin takes longer to kill and the first symptoms may not occur for half an hour, followed by a quick progression.
Even when it does not kill, sarin's effects can cause permanent harm -- damaging a victim's lungs, eyes and central nervous system.
Heavier than air, the gas can linger in an area for up to six hours, depending on weather conditions.
The most notorious attack occurred in March 1988 in Halabja when as many as 5,000 Kurds were killed and 65,000 injured when the Iraqi military used a combination of chemical agents that included sarin, mustard gas and possibly VX, a nerve agent 10 times more powerful than sarin.
It is thought to have been the worst-ever gas attack targeting civilians.
Sarin killed 13 people and injured 6,000 others when the Aum Supreme Truth cult released it in the Tokyo subway in March 1995. The cult also used the nerve agent in an attack the year before in the Japanese city of Matsumoto, killing seven.
The Syrian regime is believed to control hundreds of tonnes of various chemical agents, according to Leonard Spector of the Monterey Institute of International Studies.
In addition to blister agents known as vesicants such as mustard gas (yperite), Damascus is thought to possess sarin and possibly VX.
The Syrian regime also has the means to deliver its chemical agents, with Scud missiles, artillery shells and aerial bombs, according to defense analysts.
However, Damascus has refused to allow United Nations experts access to investigate the chemical weapons allegations despite appeals by UN chief Ban Ki-moon.
The name sarin comes from the chemists who discovered it by chance: Schrader, Ambros, Ruediger et Van der Linde. The scientists had been trying to create stronger pesticides but the formula was then taken up by the Nazi military for chemical weapons.