Oxygen is a drug. This seems like a strange statement to most people as we breathe in oxygen with every breath we take. However, the amount of oxygen we breathe in through room air is only 21%, the rest is made up mostly of nitrogen. When people are critically ill or have chronic lung disease, they often require additional oxygen to properly oxygenate the blood and tissues. In extreme cases a patient may require 100% oxygen. That is to say that they are breathing in pure oxygen.
Like all drugs, too much can be harmful and dangerous. There is still debate as to how much oxygen is too much oxygen. We do know however, high concentrations of oxygen over a period of time cause an overproduction of free radicals in the lungs. If unchecked, these radicals can severely damage or kill lung tissue. If left for a prolonged period of time the patient can suffer permanent lung damage. The general rule is that a patient should be on the least amount of oxygen necessary to keep their blood-oxygen levels high enough to properly supply the tissues and organs.