Cyanide poisoning is poisoning that results from exposure to any of a number of forms of cyanide. Early symptoms include headache, dizziness, fast heart rate, shortness of breath, and vomiting. This phase may then be followed by seizures, slow heart rate, low blood pressure, loss of consciousness, and cardiac arrest. Onset of symptoms usually occurs within a few minutes. Some survivors have long-term neurological problems.

Toxic cyanide-containing compounds include hydrogen cyanide gas and cyanide salts, such as potassium cyanide. Poisoning is relatively common following breathing in smoke from a house fire. Other potential routes of exposure include workplaces involved in metal polishing, certain insecticides, the medication sodium nitroprusside, and certain seeds such as those of apples and apricots. Liquid forms of cyanide can be absorbed through the skin. Cyanide ions interfere with cellular respiration, resulting in the body's tissues being unable to use oxygen.

Diagnosis is often difficult. It may be suspected in a person following a house fire who has a decreased level of consciousness, low blood pressure, or high lactic acid. Blood levels of cyanide can be measured but take time. Levels of 0.5–1 mg/L are mild, 1–2 mg/L are moderate, 2–3 mg/L are severe, and greater than 3 mg/L generally result in death.

If exposure is suspected, the person should be removed from the source of the exposure and decontaminated. Treatment involves supportive care and giving the person 100% oxygen. Hydroxocobalamin (vitamin B12_a) appears to be useful as an antidote and is generally first-line. Sodium thiosulfate may also be given. Historically, cyanide has been used for mass suicide and it was used for genocide by the Nazis.

If hydrogen cyanide is inhaled, it can cause a coma with seizures, apnea, and cardiac arrest, with death following in a matter of seconds. At lower doses, loss of consciousness may be preceded by general weakness, dizziness, headaches, vertigo, confusion, and perceived difficulty in breathing. At the first stages of unconsciousness, breathing is often sufficient or even rapid, although the state of the person progresses towards a deep coma, sometimes accompanied by pulmonary edema, and finally cardiac arrest. A cherry red skin color that darkens may be present as the result of increased venous hemoglobin oxygen saturation. Despite the similar name, cyanide does not directly cause cyanosis. A fatal dose for humans is on average 1.52 mg/kg of body weight, with the lowest recorded lethal dose being 0.56 mg/kg. Other sources claim a lethal dose is 1–3 mg per kg body weight for vertebrates.

Exposure to lower levels of cyanide over a long period (e.g., after use of improperly processed cassava roots; cassava is a staple food in various parts of West Africa) results in increased blood cyanide levels, which can result in weakness and a variety of symptoms, including permanent paralysis, nervous lesions, hypothyroidism, and miscarriages. Other effects include mild liver and kidney damage.

Cyanide poisoning can result from the ingestion of cyanide salts, imbibing pure liquid prussic acid, skin absorption of prussic acid, intravenous infusion of nitroprusside for hypertensive crisis, or the inhalation of hydrogen cyanide gas. The last typically occurs through one of three mechanisms:

As potential contributing factors, cyanide is present in: