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Not to be confused with the anesthetic Nitrous oxide

Nitric oxide is a free radical made up of one atom of nitrogen and one of oxygen (NO).  It is highly reactive. In the human body, it is an important part of the regulatory mechanisms of the nervous system and the immune system, among others.

It can be used for treating newborns by allowing it in breathing air.  In this form, it can reduce pulmonary hypertension.

Nitric oxide at Wikipedia