Arsenic poisoning (or arsenicosis) is a medical condition that occurs due to elevated levels of arsenic in the body. If arsenic poisoning occurs over a brief period, symptoms may include vomiting, abdominal pain, encephalopathy, and watery diarrhea that contains blood. Long-term exposure can result in thickening of the skin, darker skin, abdominal pain, diarrhea, heart disease, numbness, and cancer.

The most common reason for long-term exposure is contaminated drinking water. Groundwater most often becomes contaminated naturally; however, contamination may also occur from mining or agriculture. It may also be found in the soil and air. Recommended levels in water are less than 10–50 μg/L (10–50 parts per billion). Other routes of exposure include toxic waste sites and pseudo-medicine. Most cases of poisoning are accidental. Arsenic acts by changing the functioning of around 200 enzymes. Diagnosis is by testing the urine, blood, or hair.

Prevention is by using water that does not contain high levels of arsenic. This may be achieved by the use of special filters or using rainwater. There is no good evidence to support specific treatments for long-term poisoning. For acute poisonings treating dehydration is important. Dimercaptosuccinic acid or dimercaptopropane sulfonate may be used; but dimercaprol (BAL) is not recommended, because it tends to increase uptake of other co-occurring toxic heavy metals. Hemodialysis may also be used.

Through drinking water, more than 200 million people globally are exposed to higher-than-safe levels of arsenic. The areas most affected are Bangladesh and West Bengal. Exposure is also more common in people of low income and minorities. Acute poisoning is uncommon. The toxicity of arsenic has been described as far back as 1500 BC in the Ebers papyrus.

Ingesting large amounts of arsenic can cause symptoms similar to food poisoning, with abdominal pain, nausea, vomiting, and diarrhea starting within hours. Bloody diarrhea can cause severe fluid loss, resulting in hypovolemic shock. The heart and nervous system can also be affected, causing disruption to heart rhythms (QT interval prolongation or tachycardia), heart failure, confusion, seizures, brain swelling, coma, and death. Inhaling arsine gas – the most toxic form of arsenic – causes a multisystem disease starting 2 to 24 hours after inhalation. Symptoms include gastrointestinal distress, headache, weakness, difficulty breathing, kidney and liver dysfunction, and the destruction of red blood cells.

Chronic ingestion of lower levels of arsenic causes visible changes in the skin, typically hyperpigmentation (dark areas), but sometimes hypopigmentation (light areas) or alternating areas of each. Some experience general thickening of the skin on the palms and soles of the feet, or small thickened areas. Around 5% of those affected develop light-colored bands across the fingernail, called Mees' lines. Chronic exposure eventually causes disease across multiple body systems, including peripheral neuropathy (numbness and tingling), enlargement of the liver and spleen, diabetes, heart disease, cognitive impairment, and damage to the portal vein (non-cirrhotic portal fibrosis and portal hypertension).

Repeated arsenic exposure also increases the risk for developing several cancers, particularly of the skin, lung, liver, bladder, prostate, and blood vessels. The most common arsenic-induced skin cancer is squamous cell carcinoma in situ which typically occurs 2 to 20 years after arsenic exposure.

Arsenic poisoning is caused by incidental ingestion or inhalation of arsenic, typically from drinking contaminated well water, eating food cooked in contaminated water, or being exposed to arsenic-containing pesticides, folk medicines, or industrial chemicals. The World Health Organization considers arsenic levels above 10 parts per billion (10 micrograms per liter) to be unsafe.