JAMA wrote up a decent and very readable review about Ricin a few years ago.
Box 1 sums it up pretty well:
Ricin is a toxin derived from the castor bean plant Ricinus communis.
Poisoning can occur via ingestion, inhalation, or injection.
Ricin poisoning can have a presentation similar to gastroenteritis or respiratory illnesses.
Epidemiologic clues include increased number of patients seeking care, unexpected progression of symptoms, or a credible threat of ricin release in the community.
Person-to-person transmission does not occur.
Ricin has been procured for use as a terrorist weapon.
Inhalation and injection are considered to be the most lethal routes of exposure.
Ingestion: Mild poisoning can result in nausea, vomiting, diarrhea, and/or abdominal pain. In moderate to severe poisoning, gastrointestinal tract symptoms can progress (4-36 hours) to hypotension, liver and renal dysfunction, and possibly death.
Inhalation: Illness can occur within 8 hours and include cough, dyspnea, arthralgias, and fever, and can progress to respiratory distress and death.
Injection: Initial (ie, ≤6 hours) symptoms can include generalized weakness and myalgias; progression of illness (24-36 hours) can include vomiting, fever, hypotension, and/or multiorgan failure and death.
No clinically validated methods are available to detect ricin in biological fluids.
Analytic methods for detecting ricin (in blood) and ricinine (in urine) may be available through reference laboratories (the US Army Medical Research Institute for Infectious Diseases and the Centers for Disease Control and Prevention) in an emergency response setting.
Centers for Disease Control and Prevention and Laboratory Response Network laboratories conduct tests to detect ricin in environmental samples.
Treatment is mainly supportive and includes intravenous fluid and vasopressors (eg, dopamine) for hypotension.
Activated charcoal should be administered to persons with known or suspected ricin ingestion if vomiting has not begun and airway is secure.
Gastric lavage may be considered if ingestion has occurred in an hour or less.
If a credible threat exists, patients with illness consistent with ricin poisoning should be observed for illness progression.
The regional poison control center should be contacted for individualized care and further management.
Prevention and Reporting in the United States
All known or suspected cases of ricin exposure should be reported to the regional poison control center (222-1222) and local and state health departments.
And one point to keep in mind: Preparing and distributing ricin is a non-trivial task—particularly into a form that one could distribute by air.
The LD50 (the amount of the poison sufficient to kill 50% of those exposed) for ingested ricin for mice is about 30mg/kg. An adult human is about 70kg, so that would be 2.1 grams of ricin—that's like 10 Ibuprofen pills, enough to notice in a drink.
Mice aren't little people. The lowest dose thought to be fatal in humans by ingestion is 1mg/kg, with the highest estimate ranging up to 20mg/kg. Even 70mg of ricin would be quite a bit to dissolve secretly in a drink.
By inhalation, the dose is about a thousand-fold less— 5 micrograms per kg. But, as I noted above, preparing ricin in a form suitable for distribution by air is a much more difficult task.
My comparison of 2.1g of ricin to 10 Ibuprofen pills seems confusing to people. Ricin is only a fraction of the castor bean. Someone attempting to purify it at home seems unlikely to get anywhere close to 100% purity. Like Ibuprofen pills, ricin prepared outside of a lab or an industrial chemical company is going to be mixed with a lot of inert compounds. Hence my rough estimate of 10 or so pills worth to get 2.1g of the active ingredient. If you're looking for a lesson on metrics, tryXKCD.