Free zinc ions outside a narrow concentration range are toxic to a variety of cells in vitro.

The zinc(II) ion has recently been implicated in a number of novel functions and pathologies in loci as diverse as the brain, retina, small intestine, prostate, heart, pancreas, and immune system. Zinc ions are a required nutrient but elevated concentrations are known to kill cells in vitro. Paradoxical observations regarding zinc's effects have appeared frequently in the literature, and often their physiological relevance is unclear.