Atosiban

Tractocile, Antocin, Atosiban SUN (generic), RWJ 22164, ORF 22164

Sexual health, hormones & signalling

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Note on this profile. Atosiban is an approved hospital medicine in the EU, UK, Australia and many other countries, given by intravenous infusion to women in threatened preterm labour. It has never been approved in the United States. It is not a research-only compound and has no self-directed use. This page explains the published science.

1What it is

Atosiban is an oxytocin-receptor blocker — a tocolytic, meaning a drug given to pause labour. It was designed in the 1980s by substituting four positions of oxytocin so the molecule still docks on the receptor but no longer activates it. Ferring developed it as Tractocile; the EMA approved it in January 2000 to delay imminent preterm birth between 24 and 33 completed weeks of pregnancy. The US FDA declined approval in 1998 after an advisory committee was unconvinced by efficacy data and troubled by an imbalance in infant deaths in the placebo-controlled trial, and the company did not resubmit.

Its appeal is tolerability. Older tocolytics (beta-agonists such as ritodrine and terbutaline) cause maternal tachycardia, pulmonary oedema and metabolic effects; atosiban acts on the uterus with few systemic effects. The question the literature has never settled is whether delaying labour by 48 hours — the time needed for antenatal steroids and transfer to a neonatal unit — improves babies' outcomes, with any tocolytic.

2How it's thought to work

Oxytocin and vasopressin acting on uterine receptors release calcium from stores and drive myometrial contraction. Atosiban competes for those receptors (affinity for V1a is actually higher than for the oxytocin receptor, which may matter because vasopressin contributes to preterm contractions). Reduced calcium signalling lowers contraction frequency and amplitude. It does not treat the underlying cause of preterm labour (infection, inflammation, cervical insufficiency), which is one reason effects on prolongation are modest.

An oxytocin receptor antagonist occupying the receptor without activating it. Oxytocin released during preterm labour finds nowhere to signal and contractions are suppressed, allowing time for steroids to mature the baby's lungs.Schematic — not to scale

3What the research shows

Graded A (not superior for the key endpoints)

Versus placebo

A US multicentre of 531 women found atosiban did not significantly prolong pregnancy to 7 days overall, though it did in women ≥28 weeks; more infant deaths occurred in the atosiban arm, concentrated in babies under 26 weeks and attributed by the investigators to randomisation imbalance (Romero 2000, Am J Obstet Gynecol). This trial underlies the FDA's refusal. [1]

Versus beta-agonists

The Worldwide Atosiban versus Beta-agonists Study Group randomised 742 women: comparable rates of women remaining undelivered at 48 hours and 7 days (about 88% and 80%), with far fewer maternal cardiovascular adverse effects and discontinuations with atosiban (2001, BJOG). Smaller trials against nifedipine show similar efficacy, with nifedipine often cheaper and oral (APOSTEL III, van Vliet 2016, Lancet — no difference in perinatal outcomes). [4]

Systematic reviews

Cochrane (Flenady 2014) concluded that oxytocin-receptor antagonists did not improve neonatal outcomes compared with placebo or other tocolytics, and had fewer maternal side effects than beta-agonists; evidence quality was low to moderate. A 2022 network meta-analysis of tocolytics found most agents prolong pregnancy by 48 hours more than placebo with uncertain neonatal benefit (Wilson 2022, Cochrane Database Syst Rev †). [3]

Threatened preterm birth at 30–34 weeks (APOSTEL 8)

A Dutch–international placebo-controlled trial testing whether atosiban improves neonatal outcomes in late preterm labour completed recruitment (~750 women); published results reported no improvement in the composite adverse neonatal outcome †.

Graded A/B (mixed)

IVF embryo transfer

Trials of atosiban given around embryo transfer to reduce uterine contractions reported higher implantation in some early studies and no difference in larger RCTs (Ng 2014, Hum Reprod — no benefit in 800 women). Off-label and of uncertain value. [5]

4Reported side effects and interactions

Literature-reported

nausea (most common, ~12%), headache, dizziness, hot flushes, vomiting, tachycardia, hypotension, hyperglycaemia, injection-site reactions; rarely uterine haemorrhage or atony after birth (theoretical from receptor blockade), pulmonary oedema (mostly with concurrent beta-agonists or multiple pregnancy), allergic reactions. In the placebo-controlled trial there was a numerical excess of fetal/infant deaths at very early gestations, not confirmed elsewhere. Contraindicated before 24 weeks or after 33 completed weeks, with ruptured membranes after 30 weeks, fetal distress, antepartum haemorrhage, eclampsia or intrauterine infection.

Community-reported

essentially none documented.

Theoretical. antagonism of oxytocin receptors could in principle blunt postpartum uterine contraction if delivery follows rapidly.

Interactions. no cytochrome interactions; labetalol and betamethasone studied without clinically relevant interaction; limited data with other tocolytics (combination generally avoided).

Markers researchers commonly monitor

From the label and obstetric practice:

Marker	Why it's watched
Uterine contraction frequency (tocodynamometry) and cervical change	the direct treatment response.
Fetal heart rate monitoring	standard throughout tocolysis.
Maternal pulse, blood pressure, fluid balance, respiratory assessment	pulmonary oedema risk, mainly in multiple pregnancy or with corticosteroids.
Blood glucose	hyperglycaemia is reported, relevant alongside antenatal steroids and in diabetic women.
Signs of infection (temperature, CRP, white cell count)	chorioamnionitis is a contraindication and a common underlying cause.

Free for everyone. Safety information is never behind a paywall.

5Reconstitution and handling

Supplied as a 6.75 mg/0.9 mL solution for the bolus and 37.5 mg/5 mL concentrate for dilution into a 100 mL infusion bag. Prepared and run by hospital staff via infusion pump according to the three-step label regimen. The calculator is not applicable.

6Commonly studied alongside

Nifedipine and beta-agonists (comparator tocolytics), antenatal corticosteroids and magnesium sulphate (co-therapies the 48-hour delay is meant to enable), oxytocin (#48) and carbetocin (#216, the agonist counterpart), retosiban and barusiban (later oxytocin antagonists whose development was discontinued).

Informational cross-links. Not stack recommendations.

7Regulatory status by region

Region	Status (checked —)
United States	Not FDA-approved (application not approved 1998); not marketed
European Union	EMA centrally authorised (Tractocile, January 2000); generics authorised since 2013
United Kingdom	MHRA-licensed (Tractocile and generics); hospital use
Australia	TGA-registered (Tractocile)
South Africa	SAHPRA-registered †
Sport (WADA)	Not prohibited

8Open questions

Does any tocolytic improve neonatal outcomes, or only prolong pregnancy by hours to days?
The unexplained infant-death imbalance at very early gestations in the placebo trial.
Whether maintenance (prolonged) tocolysis after the acute episode has any value — current evidence says no.

9References

1Romero R et al. (2000). An oxytocin receptor antagonist (atosiban) in the treatment of preterm labor: a randomized, double-blind, placebo-controlled trial with tocolytic rescue. Am J Obstet Gynecol. PubMed · DOI
2Worldwide Atosiban versus Beta-agonists Study Group (2001). Effectiveness and safety of the oxytocin antagonist atosiban versus beta-adrenergic agonists in the treatment of preterm labour. BJOG. PubMed
3Flenady V et al. (2014). Oxytocin receptor antagonists for inhibiting preterm labour. Cochrane Database Syst Rev. PubMed · DOI
4van Vliet EOG et al. (2016). Nifedipine versus atosiban for threatened preterm birth (APOSTEL III): a multicentre, randomised controlled trial. Lancet. DOI
5Ng EHY et al. (2014). A randomized double blind comparison of atosiban in patients undergoing IVF treatment. Hum Reprod. PubMed · DOI
6European Medicines Agency. Tractocile EPAR — scientific discussion and current Summary of Product Characteristics. PubMed

References are being resolved to PubMed/DOI identifiers; links search PubMed for the citation until then.

† Being confirmed by our reviewers against current sources.

10Community

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Self-reported side effects Phase 2

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11Page history

31 Oct 2026Profile published — awaiting medical review

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