Entonox is a gas made up of 50% nitrous oxide and 50% oxygen. It is sometimes known as gas and air.
You control the amount of Entonox you use, but to get the best effect it is important to get the timing right. You should start breathing Entonox as soon as you feel a contraction coming on, so you will get the full effect when the pain is at its worst. You should not use it between contractions or for long periods as this can make you feel dizzy and tingly.