A blister agent, or vesicant, is a chemical compound that causes severe skin, eye and mucosal pain and irritation. They are named for their ability to cause severe chemical burns, resulting in painful water blisters on the bodies of those affected. Although the term is often used in connection with large-scale burns caused by chemical spills or chemical warfare agents, some naturally occurring substances such as cantharidin are also blister-producing agents (vesicants). Some naturally occurring substances, such as furanocoumarin, cause vesicant-like effects indirectly, for example, by increasing skin photosensitivity greatly. Vesicants have medical uses including wart removal but can be fatal if even small amounts are ingested.