Mercury can cause acute or chronic poisoning. Metallic mercury is not dangerously poisonous in very small doses: The child who bites through a clinical thermometer and swallows a few drops of the metal will probably come to no harm. However, mercuric chloride ingestion, either intentional or accidental, results in acute poisoning. The initial symptoms are nausea, vomiting, diarrhea, and abdominal pain. Cardiovascular collapse may occur after several hours.

Chronic mercury poisoning is most often due to occupational exposure or to industrial pollution of the environment. Depending on the amount and length of exposure and the age of the victim, symptoms include weakness, tremors, progressive incoordination, and paralysis. Numbness in the fingers and toes is an early sign of poisoning. Mental changes include lethargy and intellectual dullness.

Mass outbreaks of chronic mercury poisoning have occurred in the past few decades: Minamata disease in Japan was due to contamination of fish by industrial effluent. More recently, industrial contamination of fishing lakes in northern Quebec caused an epidemic of mercury poisoning, and in several countries there have been epidemics of mercury poisoning resulting from the human consumption of chemically treated wheat seed.