Mersalyl (Mersal) is an organomercury compound and mercurial diuretic. It is only rarely used as a drug, having been superseded by thiazides and loop diuretics that are less toxic because they do not contain mercury. It features a Hg(II) centre. Mersalyl was originally adapted from calomel (Hg<sub>2</sub>Cl<sub>2</sub>), a diuretic discovered by Paracelsus.