Ricin is a toxin that can be made from the waste left over from processing castor beans. It is one of the most toxic and easily produced plant toxins. If injected, as little as 500 micrograms of ricin could be enough to kill an adult; some animal research indicates that similar amounts could be lethal, if inhaled. A 500 microgram dose of ricin would be about the size of the head of a pin. Ricin has been used in the past as a bioterrorist weapon and remains a serious threat.