Arsenic

Arsenic (As) is a chemical element that is classified as a metalloid.

On Earth arsenic was first identified in the year 1250. To humans, the substance was extremely poisonous, and exposure would often lead to a painful death.

For Klingons however, arsenic was not poisonous. In fact arsenic was a necessary part of the Klingon diet, and a arsenic deficiency could lead to a number of health problems for a Klingon.