Use of Ricin: Is This Amateur Hour?

Recent incidents have brought up the subject of the biological toxin known as ricin as a possible terrorist weapon. A Utah man, one William Clyde Allen III, was arrested by the FBI for having sent letters laced with ricin to the U.S. Secretary of Defense and the Chief of Naval Operations. In June of 2018, a plot involving ricin was foiled in Germany by authorities. Ricin has a long history of attempted use as a weapon, although the large majority of instances have been unsuccessful. As this particular biological threat keeps reappearing, this post is intended to provide an overview of ricin and the threat it poses.

What is ricin?

Ricin is a toxin – a poisonous substance created in nature. It is naturally occurring and is found in castor beans and it takes its name from the botanical name (ricinus communis) of the castor bean plant. Ricin can be found as a byproduct of the manufacture of castor beans. In fact, the mash that is leftover after processing castor beans has a reasonably high ricin content. Ricin is something called a “lectin” – which means that it is a protein that binds with carbohydrates.

Ricin is extremely poisonous in animals. It acts by interfering with protein synthesis. The human body is constantly assembling things out of amino acids, and starts to malfunction if this action is interfered with. Even a small amount of ricin will cause a lot of interference. Those of a scientifically curious bent can dig into the biological and chemical actions of ricin in a much deeper way here.

In its pure form, ricin is a powder. Ricin can be injected, inhaled, and absorbed through the eyes. Unlike nerve agents, Ricin does not readily absorb through the skin, so merely handling the powder is not as dangerous as breathing it or ingesting it. However, the likely lethal doses for humans (discussed here) vary widely depending on the route of exposure.

There’s a wide margin of error here as this is mostly extrapolated from animal studies.  Scientists have estimated “LD50” doses — a dose that would kill half of those exposed. Injected, estimates are around 1 to 1.75 micrograms per kilogram of body weight. Inhaled, the amount is around 21 to 45 micrograms per kg of body weight. To put that it layman’s terms, that’s quite poisonous. Via ingestion, however, ricin is literally thousands of times less toxic, and widely variable. In food or drink, it would be 1 to 20 milligrams per kg of body weight. This could easily be the equivalent of several aspirin tablets of material, enough to throw off the taste of something. The idea that one might get killed merely by handling a letter in the post, then not washing their hands before eating seems far fetched as far as ricin is concerned, as the numbers don’t really work out.

It should be noted that claims that ricin is the “world’s most deadly poison” or similar statements should be disregarded. It is considerably less toxic than a number of other biological toxins.


Ricin poisoning is difficult to treat. Its interference with protein synthesis is irreversible. Ricin is, however, not particularly rapid-acting and the illness can last for days.

Aggressive supportive medical care can make a huge difference, particularly in borderline cases. The conventional wisdom seems to be that if a person survives more than a few days, they are likely to survive with competent medical care.

A vaccine has been developed. An antitoxin, which could revolutionize medical care for ricin victims, is in development and may be available soon. However, licensing for biological warfare vaccines and treatments poses problems in many countries. This article (pay-walled) discusses medical treatment of ricin exposure.

Is ricin a chemical weapon or a biological weapon?

The answer, technically, is that ricin is both. Toxins are chemicals. Most of them are complicated protein molecules. Yet they are of biological origin. Ricin is, therefore, covered by both the Chemical Weapons Convention and the much earlier broader Biological and Toxin Weapons Convention

Why do terrorists choose ricin?

Ricin figures prominently in the annals of amateur biological terrorism because it is the low-hanging fruit of the CBRN world. Various other chemical and biological weapons are extremely difficult to make. If one were to create a notional spectrum of difficulty of manufacture, ricin is at the easy end of the spectrum.

Both the raw material (castor beans) and the various tools and chemicals needed to extract it are widely available and generally legal, unlike many of the other options in the CBRN world. Unlike, say, sarin, ricin is likely within the technical grasp of someone capable of running a clandestine methamphetamine laboratory. It is this ease of manufacture that has allowed it to have appeared in a number of incidents and attempts involving ricin.

Is it a weapon of mass destruction?

My own opinion is that ricin is really only a “WMD” by the broad legal definition, but not by any particular properties that is has for widespread mayhem. In many ways, it is not terribly useful for purposes of mass destruction, particularly when compared to mass produced chemical weapons or more dangerous biological weapons, such as anthrax.

While ricin is quite useful as an assassination weapon by injection, this is, by definition, an individual method of dissemination, and not a wide area method. You cannot get the enemy army to line up for injections very easily.

Ricin’s toxicity by inhalation is reasonably good in theory, but actually getting ricin into the right form for inhalation is not easy. It needs to be in exactly the right particle size and the particles tend to behave poorly. There are some technical reasons for this, but I dare not get into them in detail in an open forum.

The U.S., the UK, and the USSR all investigated use of ricin as a weapon but largely did not pursue this angle due to the difficulties in making an efficient and effective dust cloud that would actually produce casualties. In fact, field experiments using cluster bombs showed that, despite its theoretical toxicity, in practical terms, ricin was only about as economical as phosgene as a battlefield weapon. Phosgene was found to be much cheaper and easier to use, so ricin work was relegated. Once serious nerve agents like Sarin became available, ricin was basically abandoned as a serious battlefield contender.

History of ricin use

Ricin has a history of use in actual and attempted killings. One of the earliest and most famous was the assassination of the Bulgarian defector Georgi Markov in 1978 by Soviet-bloc assassins using a poison-tipped umbrella. An assassination attempt in 1981 failed. A number of incidents followed in the 1990s. Rather a large percentage involved persons of alleged right wing political views in the U.S. It is not at all clear that many of the incidents (apart from a suicide attempt in Utah in 2013) in recent years have resulted in any exposure to ricin.

Although I am often loath to refer people to Wikipedia, the Wikipedia list of ricin incidents is reasonably comprehensive and has ample links to detailed articles.

Cry for help?

A reasonable number of ricin incidents in recent years have involved ricin in envelopes sent through the post. These have been intercepted by remote mail handling facilities that have been screening postal deliveries for major government installations since the 2001 anthrax terrorism incidents.

One aspect of ricin is that it is antigenic. It provokes an antibody response. This means that a technique known as immunoassay detection can be used to detect the presence of ricin. Indeed, detection of ricin is in many ways easier than many other biological warfare agents. The detection of ricin in instances of contaminated letters addressed to U.S. government recipients has been significantly reported in the media. Since ricin in an envelope has little or no mechanism to become a wide area hazard and such envelopes are readily detected during screening processes, it is hard to discern how or why someone would think that they could harm anyone by sending such a letter.

Perhaps, like many criminal acts, these instances have a pathology behind them and are not the acts of rational minds. Are they a cry for help from unstable individuals? These ricin envelopes certainly aren’t an effective weapon.

Further information

Informative fact sheets on ricin have been published by the New York State Department of Health (factsheet) and Organisation for the Prohibition of Chemical Weapons (link).


This post was revised on 22 September 2020 to provide a new link on toxicity and lethal dose figures.