Erethism, also known as erethismus mercurialis, mad hatter disease, or mad hatter syndrome, is a neurological disorder which affects the whole central nervous system, as well as a symptom complex, derived from mercury poisoning. Erethism is characterized by behavioral changes such as irritability, low self-confidence, depression, apathy, shyness and timidity, and in some extreme cases with prolonged exposure to mercury vapors, by delirium, personality changes and memory loss. People with erethism often have difficulty with social interactions. Associated physical problems may include a decrease in physical strength, headaches, general pain, and tremors, as well as an irregular heartbeat.

Mercury is an element that is found worldwide in soil, rocks, and water. People who get erethism are often exposed to mercury through their jobs. Some of the higher risk jobs that can lead to occupational exposure of workers to mercury are working in a chlor-alkali, thermometer, glassblowing, or fluorescent light bulb factory, and working in construction, dental clinics, or in gold and silver mines. In factories, workers are exposed to mercury primarily through the base products and processes involved in making the final end consumer product. In dental clinics it is primarily through their interaction and installation of dental amalgams to treat dental caries. In the case of mining, mercury is used in the process to purify and completely extract the precious metals.

Some elemental and chemical forms of mercury (vapor, methylmercury, inorganic mercury) are more toxic than other forms. The human fetus and medically compromised people (for example, patients with lung or kidney problems) are the most susceptible to the toxic effects of mercury.

Mercury poisoning can also occur outside of occupational exposures including in the home. Inhalation of mercury vapor may stem from cultural and religious rituals where mercury is sprinkled on the floor of a home or car, burned in a candle, or mixed with perfume. Due to widespread use and popular concern, the risk of toxicity from dental amalgam has been exhaustively investigated. It has conclusively been shown to be safe although in 2020 the FDA issued new guidance for at-risk populations who should avoid mercury amalgam.

Historically, this was common among old England felt-hatmakers who had long-term exposure to vapors from the mercury they used to stabilize the wool in a process called felting, where hair was cut from a pelt of an animal such as a rabbit. The industrial workers were exposed to the mercury vapors, giving rise to the expression "mad as a hatter". Some believe that the character the Mad Hatter in Lewis Carroll's Alice in Wonderland is an example of someone with erethism, but the origin of this account is unclear. The character was almost certainly based on Theophilus Carter, an eccentric furniture dealer who was well known to Carroll.