A Janus molecule is a molecule which can represent both beneficial and toxic effects. Examples are nitrous oxide

Nitrous oxide

Nitrous oxide, commonly known as laughing gas or sweet air, is a chemical compound with the formula . It is an oxide of nitrogen. At room temperature, it is a colorless non-flammable gas, with a slightly sweet odor and taste. It is used in surgery and dentistry for its anesthetic and analgesic...

 and cholesterol

Cholesterol

Cholesterol is a complex isoprenoid. Specifically, it is a waxy steroid of fat that is produced in the liver or intestines. It is used to produce hormones and cell membranes and is transported in the blood plasma of all mammals. It is an essential structural component of mammalian cell membranes...

. In the case of cholesterol, the property that makes cholesterol useful in cell membranes, namely its absolute insolubility in water, also makes it lethal. When cholesterol accumulates in the wrong place, for example within the wall of an artery, it cannot be readily mobilized, and its presence eventually leads to the development of an atherosclerotic plaque.