Examples include
carbon monoxide (CO), nitrogen monoxide (NO) and
dihydrogen monoxide (H
2O).
Carbon monoxide can be deadly for any oxygen-breathing animal, since it binds easier to the hemoglobin in your blood than oxygen. Worse, the bond is not easy to break, so the oxygen-carrying capacity of the red blood cells are permanently reduced. Evidently, large doses would render your blood useless and kill you.