Gas and Air (Entonox)

What it is

The NHS describes Entonox as "a mixture of oxygen and nitrous oxide gas". NICE specifies the ratio — "a 50:50 mixture of oxygen and nitrous oxide" — and the HSE says the same: "Entonox is half nitrous oxide and half oxygen."

It is the most widely used pain relief in labour in the UK. The Obstetric Anaesthetists' Association calls it "the most common pain relief for labour", and NICE makes it near-universal by requirement: "ensure that Entonox is available in all birth settings as it may reduce pain in labour."

The timing detail that decides whether it works

This is the part that gets missed, and it is the difference between Entonox helping and Entonox seeming useless.

The NHS: "the gas takes about 15 to 20 seconds to work, so you breathe it in just as a contraction begins. It works best if you take slow, deep breaths." The HSE gives the identical figure and the identical instruction: "it takes 15 to 20 seconds to work. It works best if you use it at the start of a contraction and take long, deep breaths."

The OAA is more precise about the whole cycle: "you should start breathing Entonox as soon as you feel a contraction coming on to get the full effect. Stop using Entonox as the contraction ends, breathing normal air between your contractions."

Wait until a contraction is at its peak to start breathing and the gas will not reach useful levels until the contraction is already fading — which is precisely the experience people describe when they say it did nothing.

ACOG's American guidance frames the timing slightly differently: "it works best when you begin inhaling 30 seconds before the start of a contraction." In practice that is the same instruction — get ahead of the pain — expressed for people who can feel a contraction building.

What it does and does not do

The NHS is careful here: "gas and air will not remove all the pain, but it can help reduce it and make it more bearable." The HSE: "using Entonox will not take the pain away completely but will reduce it. It also helps to focus your breathing."

ACOG's description is different in an interesting way. It says nitrous oxide "reduces anxiety and increases a feeling of well-being so that pain is easier to deal with", and then states plainly: "it does not numb pain." The OAA's comparison table rates Entonox as offering "moderate help", against an epidural's complete relief. All three descriptions are compatible: the value of Entonox is in taking the edge off and giving you something to do with a contraction, not in removing sensation.

Why people like it anyway

The OAA's list: it may help you feel more relaxed, it has no harmful effects on your baby, and you can use it at any time during labour. That last point is a real advantage over opioids, which the NHS says are not recommended close to the pushing stage.

Control is the other reason. The NHS: "it's easy to use and you control it yourself." You hold the mouthpiece or mask; nobody else administers it; you can stop at any moment and the effects fade quickly.

Side effects

The NHS lists feeling "lightheaded, sick, sleepy or unable to concentrate, but if this happens you can stop using it." The HSE adds that "there are no harmful side-effects to you or your baby", that it can make you feel dizzy, light-headed or a little nauseous, and that "the effects wear off very quickly" once you stop.

NICE's requirement is that women are informed "it may make her feel nauseous and light-headed" — a small honesty, but the reason people are not blindsided by it.

The OAA adds dry mouth to the list and offers the most practical mitigation of the lot: "having a break from using Entonox in between contractions helps to reduce these unwanted effects." Continuous breathing through the rest periods is what drives the queasiness.

ACOG's assessment: "nitrous oxide is safe for you and your baby. You may feel dizzy or nauseous while inhaling nitrous oxide, but these sensations go away within a few minutes."

The one caveat for a very long labour

The NHS names a limit the other sources do not: "using gas and air for more than 24 hours can reduce levels of vitamin B12 in your body, which can cause a range of issues including vitamin B12 anaemia." This is not a concern for a typical labour, but it is the reason Entonox is not simply left running indefinitely.

Combining it with other pain relief

The NHS: "if gas and air does not give you enough pain relief, you can ask for a painkilling injection as well." The OAA notes that it may also be possible to continue using gas and air alongside patient-controlled intravenous analgesia.

One restriction from NICE applies if you have had an opioid injection and want to get into water: "women should not enter water (a birthing pool or bath) within 2 hours of opioid administration or if they feel drowsy." Entonox itself carries no such restriction.

Availability: a real UK–US difference

In the UK and Ireland, Entonox is the default. NICE requires it in every birth setting, and the OAA describes it as the most common form of labour pain relief.

In the United States it is not. ACOG describes nitrous oxide as "used as a labor analgesic by some hospitals" — a qualifier that does not appear anywhere in the UK guidance. If you are giving birth in the US and want it, it is worth confirming your hospital offers it rather than assuming.

The environmental footnote

The OAA raises something the health services do not: "Entonox is a greenhouse gas and contributes to global warming. Some maternity wards are using new technology to break it down into harmless gases." It also notes that long-term exposure to high levels can affect healthcare workers, that "maternity units must track Entonox levels to keep staff safe", and that in rare cases its use may be paused if levels are too high — in which case other pain relief options are offered. This is an occupational and environmental issue rather than a safety issue for you or your baby.