A phenomenon in which a substance is harmful when used in high concentrations or in large quantities, but is beneficial to the body when used in low concentrations or in small quantities.

For example, exposure to large amounts of ultraviolet rays can cause skin cancer, but small amounts of ultraviolet rays are necessary for the body to produce active vitamin D.

The word hormesis, like the word “hormone,” comes from the ancient Greek word “hormáein” (to set in motion, to spur, to urge).