Nitrous oxide is a flavorless, odorless gas, mixed with oxygen that is inhaled through a face mask. It is not clear exactly how nitrous oxide works to reduce pain levels. Typically its use results in less pain- you may be aware of the pain without feeling bothered by it.
Your nurse or midwife will instruct you on how to use the face mask. The mask will not be attached to you while you inhale gas. This is intentional and will prevent you from inhaling too much gas at once. If you become too drowsy as a result of the gas, you will no longer be able to hold the mask to your face.
To benefit from the effects of nitrous oxide you need to place the mask on your face and breathe deeply before your next contraction begins. It takes 30 to 60 seconds for the gas to become most effective, so you should try to time your inhalations about 30 seconds before each contraction, or at the moment you begin your next contraction.
The effect of nitrous oxide has been described as a kind of strange sensation of feeling the pain while at the same time feeling a sense of bliss.