Mad Hatter's Disease

Mad Hatter’s disease, also known as mad hatter syndrome, erethism, or erethism mercurialis, is characterized by behavioral changes such as unfocused thoughts and speech, depression, irritability, insomnia, hallucinations, suicidal thoughts, forgetfulness, timidity, and difficulty in social interactions. Physical symptoms include drooling, fatigue, kidney problems, light sensitivity, headaches, tremors, rashes, skin discoloration, and irregular heartbeat.

This neurological disorder is caused by mercury poisoning. It was common among hatmakers in England in earlier centuries due to their long-term exposure to mercury which was used in making felt. The chemical was used to separate the fur from skin. Since the side-effects were still unknown during that time, many hatmakers (as well as other industrial workers) suffered from mercury poisoning which led to the expression, “mad as a hatter."