For an average adult weighing about 70 kilograms (roughly 154 pounds), a fatal dose of cyanide taken by mouth has been estimated at around 1.52 milligrams of cyanide ion per kilogram of body weight, which works out to roughly 100 milligrams total. That figure, though, is an average drawn from case reports of intentional and accidental poisonings, and the real-world range is wide enough to make any single number misleading. The form of cyanide, how it enters the body, the person’s size and metabolism, and even whether food is in the stomach all shift the lethal threshold up or down.
What the Estimated Lethal Doses Actually Look Like
Most of what we know about fatal doses in humans comes from poisoning case reports rather than controlled experiments, for obvious ethical reasons. The Agency for Toxic Substances and Disease Registry (ATSDR) compiled those case reports along with animal data to arrive at several estimates. For cyanide taken by mouth, the average lethal dose is about 1.52 mg of cyanide ion per kilogram of body weight. But some fatal cases involved absorbed doses as low as 0.7 mg per kilogram, meaning a person on the lighter or more vulnerable end of the spectrum could die from a smaller amount than the average suggests.1Agency for Toxic Substances and Disease Registry. Toxicological Profile for Cyanide – Section: 2.2 DEATH
The specific compound matters too. Hydrogen cyanide, the gas form, has an estimated lethal range of 0.67 to 3.37 mg of cyanide ion per kilogram for a 70-kg person. Potassium cyanide, a solid salt more commonly encountered in poisoning cases, falls in a tighter range of 0.86 to 1.43 mg per kilogram. These numbers reflect the fact that different cyanide compounds release their cyanide ion at different rates and with different efficiency once inside the body.1Agency for Toxic Substances and Disease Registry. Toxicological Profile for Cyanide – Section: 2.2 DEATH
For inhaled hydrogen cyanide gas, the math works differently. Rather than a dose measured in milligrams swallowed, toxicologists describe a concentration in the air multiplied by the time spent breathing it. The estimated average airborne concentration that would kill a person within 30 minutes is about 622 parts per million (ppm). At higher concentrations, death can come in minutes or even seconds. At lower concentrations, a person might survive long enough for symptoms to develop more gradually, but the window for treatment is still extremely narrow.1Agency for Toxic Substances and Disease Registry. Toxicological Profile for Cyanide – Section: 2.2 DEATH
How the Route of Exposure Changes Everything
Cyanide can enter your body through the lungs, the digestive tract, or even the skin, and each route produces a dramatically different timeline. Breathing in hydrogen cyanide gas is the fastest path to a crisis. Symptoms can begin within seconds of inhaling a high concentration, because the gas crosses from lung tissue into the bloodstream almost instantly. This is the scenario firefighters and industrial workers worry about most, and it is why hydrogen cyanide gas is considered one of the most acutely dangerous substances a person can encounter.2NCBI Bookshelf. Cyanide Toxicity – Section: Toxicokinetics
Swallowing a cyanide salt like potassium cyanide or sodium cyanide is slower but still fast by poisoning standards. Symptoms typically appear within minutes, though in some cases the onset can stretch to a few hours depending on what else is in the stomach and how much was ingested. An empty stomach speeds absorption; a full stomach slows it slightly. The acidic environment of the gut converts cyanide salts into hydrogen cyanide, which is then absorbed through the stomach and intestinal lining.
Skin absorption is the least well documented route. There are case reports of people developing symptoms after prolonged skin contact with cyanide solutions, with onset ranging from minutes to hours. The data on dermal exposure are limited enough that toxicologists are cautious about estimating a lethal skin dose, but the takeaway is clear: cyanide does not need to be swallowed or inhaled to be dangerous.2NCBI Bookshelf. Cyanide Toxicity – Section: Toxicokinetics
What Cyanide Does Inside the Body
The reason cyanide kills at such small doses is that it attacks the most fundamental energy-producing process in every cell. Your cells rely on a chain of chemical reactions to convert oxygen and nutrients into usable energy. Cyanide binds to a key enzyme in that chain, cytochrome c oxidase, and shuts it down. With that enzyme blocked, cells cannot use oxygen even though plenty of oxygen is circulating in the blood. The result is a kind of internal suffocation: the blood is oxygen-rich, but the tissues are starving for energy.3NCBI Bookshelf. Cyanide Toxicity – Section: Pathophysiology
This explains why high-dose cyanide exposure can kill within minutes. The brain and heart consume the most oxygen of any organs, so they are the first to fail when cells can no longer use oxygen. Seizures, cardiac arrest, and loss of consciousness follow in rapid succession. At lower but still dangerous doses, the progression is similar but slower, giving more time for symptoms like headache, confusion, rapid breathing, and nausea to develop before the situation becomes critical.
One detail that sometimes surprises people: cyanide poisoning can make the skin appear unusually pink or cherry-red, because the blood remains saturated with oxygen that the tissues cannot extract. In most other forms of suffocation, the skin turns blue from oxygen depletion. The paradox of oxygen-rich blood in an oxygen-starved body is a hallmark of cyanide poisoning and a clue that emergency physicians are trained to recognize.
Why Some People Are More Vulnerable Than Others
The lethal dose estimates described above are averages across reported cases, but individual vulnerability varies considerably. The most obvious factor is body mass. A larger person has more tissue to absorb and dilute a given dose of cyanide, so the per-kilogram threshold matters more than any absolute number of milligrams. A 50-kilogram person and a 100-kilogram person facing the same absolute dose are in very different situations.
Beyond size, the body has a built-in cyanide detoxification system. An enzyme called rhodanese converts cyanide into thiocyanate, a much less toxic compound that the kidneys can then excrete. People vary in how much rhodanese activity they have, and that variation affects how quickly their bodies can neutralize a small cyanide exposure before it overwhelms the system.4US EPA. Toxicological Review of Hydrogen Cyanide and Cyanide Salts – Section: Susceptible Populations
Children are more susceptible than adults for reasons that go beyond their smaller size. Their metabolic rates are higher relative to body mass, meaning they consume more oxygen per kilogram and are hit harder when oxygen utilization shuts down. People with preexisting cardiovascular or respiratory conditions may also tolerate cyanide exposure less well, since their baseline oxygen delivery is already compromised. Nutritional status plays a role too: the rhodanese detoxification pathway requires sulfur donors, and people with poor diets or chronic malnutrition may have a diminished capacity to neutralize cyanide.
Chronic low-level exposure creates its own set of problems distinct from acute poisoning. People who regularly consume cyanide-containing foods without adequate nutrition may develop a condition called konzo, a form of irreversible paralysis seen primarily in sub-Saharan Africa among populations that depend heavily on improperly processed cassava. The mechanism differs from acute poisoning; chronic exposure overwhelms the detoxification system gradually rather than all at once.
Cyanide in Everyday Foods
Cyanide is not just an industrial chemical or a poison-mystery trope. It occurs naturally in dozens of common plants, locked inside compounds called cyanogenic glycosides. When these compounds are crushed, chewed, or metabolized, they release hydrogen cyanide. The European Food Safety Authority has documented fatal poisoning cases from ingestion of amygdalin preparations, bitter almonds, and cassava.5EFSA Journal. Evaluation of the health risks related to the presence of cyanogenic glycosides in foods other than raw apricot kernels – Section: Summary
Cassava is probably the most significant dietary source of cyanide worldwide. The root is a staple food for hundreds of millions of people in tropical regions, but it contains enough cyanogenic glycosides that improper preparation can lead to poisoning. Traditional processing methods, such as prolonged soaking, fermentation, and thorough cooking, break down most of the cyanide-releasing compounds. Problems arise when food shortages or time pressures lead people to skip or shorten these steps.
Bitter almonds are another well-known source. A single bitter almond contains enough amygdalin to release several milligrams of hydrogen cyanide when chewed and digested. The sweet almonds sold in grocery stores are a different variety with far lower cyanogenic glycoside content, which is why eating a handful of sweet almonds is perfectly safe. Bitter almonds, by contrast, have been the subject of regulatory warnings in multiple countries, and consuming even a modest number of them raw can produce symptoms of cyanide poisoning.
Apricot kernels attracted public attention after being marketed as a supposed cancer treatment. The kernels contain amygdalin, which some alternative medicine advocates rebranded as “laetrile” or “vitamin B17.” There is no credible evidence that amygdalin treats cancer, and the doses recommended by proponents of the therapy have caused documented cases of cyanide poisoning. Several countries now limit the sale of raw apricot kernels or require warning labels.
Apple seeds and cherry pits also contain cyanogenic glycosides, a fact that generates a lot of anxiety on the internet. In practice, the amount of cyanide in a few accidentally swallowed apple seeds is negligible. The seeds have a tough coating that resists digestion, and even if you chewed them thoroughly, you would need to consume a very large quantity to approach a dangerous dose. The risk from these common fruit seeds is effectively zero under normal eating conditions.
Common Misconceptions About Cyanide Lethality
Pop culture has given cyanide a reputation as an instant, guaranteed killer, the spy’s capsule that brings death in seconds. The reality is more complicated. While high-concentration inhaled hydrogen cyanide gas can indeed kill within minutes, swallowed cyanide salts are not always immediately fatal. Case reports include people who survived oral cyanide ingestion because they vomited quickly, received antidote treatment in time, or simply happened to have ingested a dose that was serious but not quite lethal for their body size. Survival is not common with large intentional doses, but it is not unheard of either.
Another misconception is that cyanide has a fixed lethal dose the way you might think of a switch being flipped. In toxicology, lethal dose estimates are statistical constructs. The commonly cited “LD50” for a substance is the dose expected to kill half of a test population, not the dose that kills every individual. Some people in the population would die from less; others would survive more. The wide range in the ATSDR estimates, from 0.67 to 3.37 mg per kilogram for hydrogen cyanide, reflects this variability.1Agency for Toxic Substances and Disease Registry. Toxicological Profile for Cyanide – Section: 2.2 DEATH
There is also a persistent belief that you can smell cyanide and therefore detect it before reaching a dangerous exposure. Hydrogen cyanide gas does have a faint bitter-almond odor, but the ability to detect that odor is genetically variable. A significant portion of the population cannot smell it at all, and even among those who can, the detection threshold may be above the concentration that causes symptoms. Relying on smell as a warning system for cyanide exposure is dangerous.
Industrial and Environmental Exposure Scenarios
Outside of deliberate poisoning and dietary sources, most human cyanide exposure occurs in occupational settings. Industries that use cyanide compounds include metal electroplating, gold and silver mining, chemical manufacturing, and certain photographic processes. Workers in these environments face the risk of inhaling hydrogen cyanide gas or absorbing cyanide solutions through the skin, which is why workplace exposure limits are set far below acutely dangerous concentrations.
House fires are a less obvious but significant source of cyanide exposure. When synthetic materials like nylon, polyurethane foam, and certain plastics burn, they release hydrogen cyanide along with carbon monoxide and other toxic gases. Firefighters and people trapped in burning buildings can be exposed to cyanide concentrations high enough to cause poisoning. The combination of cyanide and carbon monoxide is particularly dangerous because both toxins impair oxygen utilization through different mechanisms, and together they create a worse outcome than either alone would predict.3NCBI Bookshelf. Cyanide Toxicity – Section: Pathophysiology
Smoke inhalation casualties are sometimes treated empirically for cyanide poisoning even before lab confirmation, because waiting for blood cyanide levels to come back takes time that the patient may not have. The decision to administer an antidote in a fire victim with altered consciousness and metabolic signs of poisoning is a judgment call made in the field or the emergency department, and it illustrates a broader truth about cyanide toxicity: by the time you have confirmed the exposure, the window for effective treatment may already be closing.
How Antidotes Work Against Cyanide
Two main antidote strategies exist. The first, used for decades, involves giving compounds that create a form of hemoglobin called methemoglobin, which binds cyanide and pulls it away from the enzymes it is poisoning. Sodium nitrite followed by sodium thiosulfate is the classic combination. The nitrite generates methemoglobin to scavenge cyanide, and the thiosulfate provides the sulfur donors that the body’s own rhodanese enzyme needs to convert cyanide into the relatively harmless thiocyanate. This approach works but has drawbacks: sodium nitrite itself reduces the blood’s oxygen-carrying capacity, which is a problem when the patient may also be suffering from carbon monoxide poisoning.
The second strategy uses hydroxocobalamin, a form of vitamin B12 that binds directly to cyanide to form cyanocobalamin, which the kidneys excrete. Hydroxocobalamin does not interfere with oxygen transport, making it safer for smoke inhalation victims who may have carbon monoxide poisoning simultaneously. It has become the preferred first-line antidote in many emergency systems for exactly that reason, though its high cost and the large volumes required for an effective dose remain practical barriers in some settings.
Both antidotes are most effective when given early. Cyanide’s speed of action means that a delay of even 15 to 30 minutes can be the difference between full recovery and permanent brain damage or death. For people who work around cyanide or respond to fires, the availability and proximity of antidote kits is a genuine life-or-death logistics question, not a bureaucratic detail.