Cyanide: why tiny amounts can be fatal

Cyanide is a highly dangerous poison that can be fatal in very small amounts. Because it acts extremely quickly, hydrogen cyanide gas was historically used in executions during wartime.

There are two main forms people should be aware of:

• Solid cyanide salts, such as sodium cyanide and potassium cyanide
• Hydrogen cyanide gas (HCN), which can form when acids such as hydrochloric acid or sulphuric acid react with cyanide salts

Both forms are extremely toxic and potentially deadly.

What is cyanide and how toxic is it?

Cyanide refers to a group of chemicals that all contain the cyanide ion (CN⁻). The most toxic form in the body is the “free” form, known as hydrogen cyanide (HCN).

Acute cyanide poisoning can cause:

• severe breathing difficulty
• seizures
• loss of consciousness

The main organs affected are:

• the central nervous system
• the heart and circulatory system
• the liver
• the kidneys

Only a very small amount is needed to be lethal to humans – just a few milligrams per kilogram of body weight – which is why cyanide is regarded as particularly dangerous.

Where cyanide is found

Cyanide compounds are often found as:

• white solid alkali metal cyanide salts
• heavy-metal cyanide compounds

Cyanide can also occur naturally, especially in some plants, often in the form of hydrocyanic acid (hydrogen cyanide). In laboratories, it can be detected and measured by distillation techniques and analysis of cyanide ions.

How cyanide affects the body

Cyanide can enter the body through several routes:

• breathing in hydrogen cyanide gas
• swallowing cyanide in tablet or liquid form
• absorbing cyanide through the skin in certain circumstances

Once in the body, cyanide interferes with how cells use oxygen. It blocks normal cellular respiration, meaning cells cannot produce energy. If the dose is high enough, this quickly leads to failure of vital organs and can result in death.

Speed of onset

If cyanide is swallowed on an empty stomach, symptoms can appear within minutes.

If taken with food, symptoms may be delayed for hours because digestion slows absorption.

When cyanide salts are ingested, stomach acid can convert them into hydrogen cyanide gas inside the stomach, which is then rapidly absorbed.

Inhaling high concentrations of hydrogen cyanide gas, especially in small, poorly ventilated spaces, can cause collapse and death very quickly.

Lower concentrations may not kill immediately but can still cause serious, potentially life-threatening poisoning.

Symptoms of cyanide poisoning

Cyanide poisoning can be acute (large or rapid exposure) or chronic (small doses over a long period).

1. Acute cyanide poisoning (large or rapid exposure)

Acute poisoning occurs when a person is exposed to a large amount of cyanide in a short time. Symptoms usually appear very quickly – sometimes within minutes – and can lead to sudden death if not treated urgently.

Nervous system symptoms:

• headache
• dizziness
• confusion
• agitation or restlessness
• loss of consciousness
• seizures
• Breathing and heart symptoms:
• difficulty breathing or shortness of breath
• slow or laboured breathing
• palpitations or irregular heartbeat
• low blood pressure
• slow heart rate or abnormal heart rhythm
• respiratory arrest and cardiac arrest

General symptoms:

• nausea and vomiting
• marked weakness or collapse
• Skin and mucous membranes:
• skin and lips may appear unusually pink or red, even though the body is severely starved of oxygen

2. Chronic cyanide poisoning (small doses over time)

Chronic poisoning happens when small amounts of cyanide are taken in repeatedly over a long period. The symptoms are often vague and non-specific, making it harder to recognise.

Possible symptoms include:

• persistent headache
• drowsiness or fatigue
• nausea and vomiting
• red or irritated rash on the skin

If long-term exposure is not treated, more serious symptoms can develop, such as:

• dilated pupils
• cold body temperature or cold skin
• abnormal heart rhythm
• in severe cases, collapse and death

Important: Any suspected cyanide exposure is a medical emergency. If someone develops sudden breathing problems, confusion, collapse, or seizures after possible contact with cyanide or toxic smoke, they need immediate emergency medical help.