It's such a swift, deadly poison that it's been the focus of some strange, rare murders throughout history.
It's not clear how much of the substance was inside the envelopes sent to Capitol Hill, which never reached their intended targets thanks to a heavy duty mail-interceptor system put in place after the 2001 anthrax attacks.
But it was a close call, and police have taken custody of a suspect: Kevin Paul Curtis, a Mississippi man whom a local newspaper says is an award-winning celebrity impersonator.
Inhaling ricin can cause difficulty breathing, fever, cough, nausea, and tightness in the chest leading to potentially fatal respiratory failure, according to the Centers for Disease Control and Prevention.
Why would the person behind the letters have picked ricin as his poison of choice?
For starters, it's relatively easy to get. Ricin is made from the seeds of castor beans, which are commonly grown.
And the stuff can be deadly no matter how it gets into your body, whether by ingestion, inhalation or injection. It only takes inhaling a tiny amount—1.78mg or the size of a few grains of table salt—to kill an average adult.
There is no known antidote.
The US government is well aware of the deadly nature of ricin. A report prepared for Congress in 2010 warned that ricin might be used in terrorist attacks.
"Persons exposed to ricin exhibit different symptoms depending on the route of exposure," it said. "Ingestion of ricin causes nausea, vomiting, diarrhea, gastric hemorrhaging and shock. With a sufficient dose, death occurs within three to five days. Injection of ricin produces severe internal bleeding and tissue death, which can result in the collapse of major organ systems. Death often follows such a collapse."
The most famous ricin poisoning is the famous ricin umbrella assassination of Bulgarian dissident Georgi Markov in 1978.
The Bulgarian secret service, with help from the KGB, stabbed Markov in the leg with an umbrella as he was walking across a London bridge. A tiny ricin pellet in the umbrella went into his bloodstream and he died three days later.
Ricin was one of the "weapons of mass destruction" that former Secretary of State Colin Powell claimed Saddam Hussein was stockpiling in his now infamous speech to the United Nations Security Council in February 2003.
A Midwestern militia group, the Minnesota Patriots Council, cooked up batches of ricin in the 1990s after ordering castor beans from a right-wing magazine. The group was plotting to use the poison to kill U.S. marshals, IRS agents and deputy sheriffs in the community.
Another set of ricin letters was sent to the US government in 2003, signed by a "Fallen Angel". No one was ever caught and the ricin never reached its intended target.
Ricin also mysteriously turned up in a Las Vegas hotel room in 2008 next to a comatose unemployed computer graphics artist. Roger von Bergendorff had inhaled some ricin he apparently made himself. He was found with enough of the poison on his person to kill hundreds of people.
The recent ricin letters serve as a reminder of how easily accessible some dangerous toxins are. But their interception is proof that the Capitol's mail screening system is working pretty well, too.