Nitrous oxide is a sedative gas that is used for dental treatment. It is a mild relaxant, and is very helpful if you find dental treatment stressful
If you have true dental phobia, this will probably not be strong enough for you, and you will need sedative medications taken by mouth.
Nitrous oxide is breathed through a mask placed over your nose. The dentist usually starts out with a low concentration and then gradually increases it until you feel the desired effect. While you're receiving the gas, it's good to give the dentist feedback on its effects. You can strengthen its effect by breathing more deeply. You can also lessen the effect by breathing more through your mouth. But it's best to let the dentist adjust the amounts, because he or she will take note of the concentration that works best for you and will then be better able to treat you.