Two drugs, opposite timing

These are usually described together because they are both opioids, but as options in labour they behave very differently.

Pethidine is a single injection into your thigh or buttock. The NHS: "it takes about 20 minutes to work after the injection. The effects last between 2 and 4 hours." The HSE gives the same figures. Once it is in, it is in.

Remifentanil goes into a vein in your arm and you control it. The NHS: "you control it yourself by pushing a button. It works quickly and wears off after a few minutes. You can use it up until your baby is born."

That contrast drives most of the practical differences below. Pethidine is a long, blunt instrument you commit to. Remifentanil is short-acting and titrated by you, contraction by contraction.

How much pain relief to expect

NICE sets expectations low and does so deliberately: "ensure that pethidine, diamorphine or other opioids are available in all birth settings. Inform the woman that these will provide limited pain relief during labour and may have significant side effects."

The HSE's framing is that pethidine "may help lessen your pain, especially if used with other methods", and the NHS notes it "can also help you to relax". The OAA puts it similarly: "some women say the opioid injection makes them feel more relaxed and less worried about the pain."

Relaxation rather than analgesia is a fair summary of what people report. If you are expecting the pain to stop, this is not the option that does that.

Side effects for you

The NHS on pethidine and diamorphine: "it can make you feel woozy, sick and forgetful." The HSE: "the injection can make you feel drowsy, dizzy or sick." NICE names drowsiness, nausea and vomiting, and requires that "if an intravenous or intramuscular opioid is used, also administer an antiemetic" — the anti-sickness medicine is built into the recommendation, not an afterthought.

For remifentanil, the NHS lists sleepy, sick, dizzy or itchy, and notes that these "stop once you stop taking the medicine." It also flags the monitoring: "you'll need a small clip on your finger to measure your oxygen levels, as remifentanil can make you feel breathless or need oxygen."

The OAA's list for patient-controlled intravenous analgesia is blunter about the mechanism: it can slow your breathing, cause your oxygen levels to drop, make you itchy, sick or sleepy. On injected opioids it adds that "high doses of opioids can slow down your breathing. This is very rare with the doses given in labour."

ACOG's US summary of systemic opioids: "side effects are minor and include itching, nausea, vomiting, feeling drowsy, or having trouble concentrating... High doses of systemic analgesics can cause you to have breathing problems."

Effects on the baby

This is the part that determines when the drugs can be given.

NICE requires women to be told that opioids "may have significant side effects... for her baby (for example, short-term respiratory depression and drowsiness, which may last several days and may make it more difficult to breastfeed)."

The NHS puts the timing rule around it: pethidine "would not be recommended if you're getting close to the pushing (second) stage of labour", and "if pethidine or diamorphine are given too close to the time of delivery, they may affect the baby's breathing — if this happens, another medicine to reverse the effect will be given." It also states directly that "these medicines can interfere with the baby's first feed." The HSE agrees: "if given too close to birth, your baby may be a bit sleepy after birth."

ACOG describes the same constraint from the other side: "you may not be able to get systemic analgesics within the hour before delivery."

For remifentanil, the NHS notes it "can affect the baby's breathing but this usually wears off quickly", and the OAA states that patient-controlled intravenous analgesia "is considered safe for babies". The short half-life is the reason it can be used right up to birth when pethidine cannot.

Where each can be given

Pethidine is available anywhere. NICE requires opioids to be available in all birth settings, which includes home births and freestanding midwifery units.

Remifentanil is not. NICE is unambiguous: "only use remifentanil PCA in obstetric units because of the risk of respiratory depression in women that may need anaesthetic support." The OAA adds a further caveat — "not every labour ward can safely offer PCIA pain relief. Ask your midwife what is available in your hospital." It is worth asking before you build a birth plan around it.

The dose and the off-label status

NICE's recommendation is specific: "consider intravenous remifentanil patient-controlled analgesia (PCA), at 40 micrograms per bolus with a 2-minute lockout period, as an option for women who want ongoing pain relief during labour and birth." It then adds a disclosure that is easy to miss: "in September 2023, this was an off-label use of remifentanil."

Off-label means the medicine is being used outside the terms of its marketing authorisation — a routine and legal practice in obstetrics, but something you are entitled to know about and ask about before consenting.

How remifentanil compares with pethidine

NICE is one of the few sources to publish a direct comparison, and it favours remifentanil. Compared with intramuscular pethidine, with 40 micrograms of remifentanil women are:

"less likely to need an epidural or have a birth using forceps or ventouse"
"more likely to have a spontaneous vaginal birth or need supplemental oxygen"

The supplemental oxygen is the trade-off, and it is why the finger clip and the obstetric-unit restriction exist. NICE asks clinicians to discuss both sides and "help them make a supported decision about its use."

The HSE also flags that pethidine "is not suitable for some people. For example, people with a history of epilepsy" — a reason to raise your medical history early rather than at the point of asking for it.

Combining them with other options

The NHS notes you can ask for a painkilling injection in addition to gas and air, and the OAA says "it may be possible to continue to use gas and air as well as your PCIA."

One restriction is firm and catches people out. NICE: "women should not enter water (a birthing pool or bath) within 2 hours of opioid administration or if they feel drowsy." If a birth pool matters to you, the order in which you take pain relief matters too — an opioid injection closes the pool for the next two hours.

What is worth knowing before you decide

Opioids in labour sit awkwardly between the self-help options and an epidural: more than the former, considerably less than the latter, with side effects that are real for both of you. NICE's own language — "limited pain relief" and "significant side effects" — is the most useful summary anyone has written.

The OAA's comparison table makes the same point structurally: an epidural is rated as giving complete pain relief, while an opioid injection sits alongside gas and air in the moderate band. If your reason for choosing an injection is that an epidural feels like too big a step, it is worth knowing you are not choosing a smaller version of the same thing.

Pethidine and Remifentanil