As a weapon of terror, experts say ricin (search) has two big advantages: It is easy to make and frighteningly poisonous.

However, it is not nearly as worrisome as anthrax (search) and a short list of other biologically derived hazards and is unlikely to be used to cause large numbers of deaths.

Ricin is derived from castor beans (search). The poison is made from the waste left over when the beans are processed for their oil.

"It is easy to make. You can do it in your kitchen from castor beans," said Dr. Donna Seger of Vanderbilt University, president of the American Academy of Clinical Toxicology (search).

Though one of the most toxic natural substances, ricin poisoning is very rare and occurs mostly from accidentally chewing castor beans. One or two beans can be enough to cause death.

Ricin is most deadly if injected. Just 500 micrograms — an amount about the size of the head of a pin — can kill. In the most famous case of ricin poisoning, the Bulgarian dissident Georgi Markov (search) was killed in London in 1978 when jabbed with an umbrella that injected a ricin pellet under his skin.

Ricin can also be deadly if breathed. However, relatively large amounts are needed to do this. For instance, Seger points out that it would take hundreds of pounds of ricin to do as much damage this way as a couple of pounds of anthrax.

"It has always been a me-too in bioterrorism preparedness, a close second to the top tier," said Dr. Scott Lillibridge, director of the Center for Biosecurity and Disaster Preparedness at the University of Texas Health Science Center in Houston.

In 1999, the Centers for Disease Control and Prevention drew up a list of the most dangerous potential bioweapons: smallpox, anthrax, botulism, viral hemorrhagic fevers, plague and tularemia.

"It didn't make the A-list," said Lillibridge, who was the CDC's bioterrorism preparedness chief at the time. Nevertheless, "it always comes up in law enforcement and intelligence discussions because of its ease of access and development."

Experts say that ricin is probably most dangerous in small-scale, even one-on-one attacks and unlikely to ever be used in the kind of huge atrocities that are feared with smallpox.

"I don't believe ricin will ever be a weapon that will be formulated to cause large numbers of deaths or illness," said Dr. Michael Osterholm, a bioterrorism authority at the University of Minnesota.

Experts say that military analysis have reviewed the potential effects of releasing ricin into the air during battle. "In those situations, it would require the release of such a large amount that it is not felt to be a feasible weapon to kill people," said Dr. Greg Evans, bioterrorism chief at St. Louis University.

He noted that sprinkling ricin on the food in a salad bar could make many sick and kill some. But trying for a population-wide attack by poisoning food in a processing plant would be another matter.

"You would have to back a truck in there to get enough mixed in with large amounts of food so it wouldn't be diluted out," Evans said.

Symptoms of ricin poisoning usually develop within a few hours, depending on the means of exposure.

If inhaled, typical first symptoms would be difficulty breathing, fever, coughing and nausea. This could be followed by fluid buildup in the lungs, low blood pressure and eventual death.

People who swallow ricin would probably develop vomiting and diarrhea. This could lead to severe dehydration and low blood pressure.

There is no antidote for ricin. Treatment is supportive care, such as intravenous fluids and blood pressure medicine.