Nitric oxide, molecular model. Nitric oxide, or nitrogen monoxide, acts as a free radical and has important uses in the chemical industry. It is a cellular signaling molecule, and is used as a vasodilator drug in medicine. Nitric oxide gas has shown promise as a treatment for covid-19, the respiratory disease caused by the coronavirus SARS-CoV-2 (previously 2019-nCoV), which was first identified in Wuhan, China, in December 2019. Atoms are represented as spheres and are colour-coded: nitrogen (blue) and oxygen (red).