Entonox is a colourless gas, supplied piped or in cylinders. If piped, the tubing is a blue and white stripe, if supplied in a cylinder, the cylinder shoulders are blue and white. Newer cylinders have the name written on the side. For the purposes of analgesia, Entonox is self-administered by the woman, under the supervision of an appropriately trained midwife. In the UK Entonox is on the P (Pharmacy) list of medicines (see Chapter 18) and so may be administered by midwives in the course of their professional practice. As for any medication, the midwife must be satisfied that its use is indicated and that they have been trained accordingly. The woman uses a mouthpiece or mask to which a demand/expiratory valve is attached. The mask is held over the nose and mouth with an airtight seal, or the mouthpiece is placed in the mouth. As the woman breathes in, the Entonox is heard to be released; the apparatus should remain in place during expiration. Self-administration allows the woman to regulate the amount taken according to need and to avoid overdosing. The apparatus also includes a microbiological filter to prevent any cross-infection. It has a slightly sweet smell and taste (Nagele et al 2014) but often not enough for women to notice or comment.

Analgesia can be achieved within a few breaths (25–35 seconds), the maximum peak occurring after only 2–3 minutes of breathing it. For the most effective use of Entonox, the contractions are palpated so that the gas is taken immediately when the contraction commences, and before the perception of pain. Just as the effects are seen rapidly, the gas is also excreted from the lungs rapidly. This means that the effects are present for approximately 60 seconds after breathing it has ceased, with no effects after that, until breathing it recommences. Consequently the woman can stop inhaling the gas at the peak of the contraction, knowing that the analgesic effect is still in place. This is also one way to avoid an excessive intake of Entonox (see below).

It is a suitable analgesic for labour, often giving sufficient pain relief while allowing the woman to retain control within her labour. Her ability to experience contractions remains, as does her level of consciousness, rationality and mobility. NICE (2014) suggest that it should be available in all birth settings, although it is acknowledged that its effectiveness is not proven. Green (1993) indicates that women have good levels of satisfaction with Entonox use in labour. From their observations in practice many midwives will agree that one of the benefits of inhalational analgesia is that the woman focuses on breathing regularly, has something to hold on to and therefore has a certain level of distraction that may help her through each contraction.

In the second stage of labour breathing the analgesia may help the woman in the interim if waiting for the presenting part to descend before actively pushing. It can be used effectively for examining the perineum and for suturing at the end of the third stage of labour. Without uterine contractions the woman is able to breathe the Entonox for a minute or more before any part of the procedure is undertaken.

It is noted that Entonox crosses the placenta, but it has no known negative effects on the fetus (BOC 2011). The neonate also excretes it via their lungs at birth, this effective form of excretion avoids the less mature liver and kidneys (Jordan 2010).