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approvedHormone Regulation

Atosiban

Also known as: Tractocile, RWJ-22164, Antocin, Oxytocin Antagonist

Atosiban (Tractocile) is a synthetic nonapeptide that acts as a competitive antagonist at both oxytocin (OT) and vasopressin V1a receptors. Approved in the European Union and many other countries (but not the US) for the acute treatment of preterm labor in pregnant women between 24 and 33 weeks of gestation, atosiban inhibits oxytocin-driven uterine contractions without the significant maternal and fetal side effects associated with beta-agonist tocolytics. It has been in clinical use since 2000 and has an excellent safety profile, making it the preferred tocolytic in European clinical practice.

3 cited references·5 researched benefits

Quick Answer

Atosiban (Tractocile) is a synthetic peptide that blocks oxytocin and vasopressin V1a receptors to inhibit premature uterine contractions. Approved in the EU since 2000 for acute tocolysis in preterm labor (24-33 weeks), it delays delivery by 48 hours to allow corticosteroid administration for fetal lung maturation. It has fewer maternal cardiovascular side effects than beta-agonists like ritodrine, though it is not FDA-approved in the United States.

Key Facts

Mechanism
Atosiban competitively antagonizes both the oxytocin receptor (OXTR) and the vasopressin V1a receptor on myometrial smooth muscle cells. By blocking oxytocin binding to OXTR, atosiban prevents the Gq-mediated signaling cascade that activates phospholipase C, IP3 production, and intracellular calcium release — the pathway responsible for myometrial contraction. V1a receptor blockade provides additional tocolytic benefit, as vasopressin also contributes to myometrial contractility. The dual antagonism produces effective uterine quiescence without affecting other smooth muscle beds, explaining atosiban's favorable side effect profile compared to non-selective tocolytics. Atosiban has negligible affinity for V2 receptors, so it does not affect renal water handling.
Research Status
approved
Half-Life
~18 minutes (initial phase)
Molecular Formula
C₄₃H₆₇N₁₁O₁₂S₂
Primary Use
Hormone Regulation
Table of Contents

Benefits

  • Effective tocolysis in preterm labor — delays delivery by at least 48 hours in 60-70% of patients, enabling corticosteroid administration for fetal lung maturationstrong
  • Superior maternal safety profile — significantly fewer cardiovascular side effects compared to beta-agonists (ritodrine, terbutaline) including less tachycardia, hypotension, and pulmonary edemastrong
  • Minimal fetal exposure — does not cross the placenta in significant amounts, resulting in no direct fetal cardiovascular or metabolic effectsstrong
  • No significant effect on maternal glucose, insulin, or electrolytes — unlike beta-agonists, making it safe in gestational diabetes and cardiac diseasestrong
  • Being researched for improving IVF implantation rates — may reduce uterine contractions during embryo transfer, potentially improving pregnancy ratespreliminary

Dosage Protocols

RouteDosage RangeFrequencyNotes
Intravenous bolus + infusion (standard protocol)6.75 mg bolus, then 18 mg/hr for 3 hours, then 6 mg/hr for up to 45 hoursSingle treatment course up to 48 hoursTotal maximum dose is 330 mg per treatment course. Bolus given over 1 minute. May be repeated if contractions recur, but benefit of repeated courses is not well established.

Medical disclaimer

Dosage information is provided for educational reference only. Always follow your prescriber's instructions and consult a qualified healthcare provider before starting any peptide protocol.

Side Effects

  • Nausea — the most common side effect, reported in 10-12% of patients in clinical trialscommon
  • Headache — reported in 5-10% of patientscommon
  • Injection site reactions — local pain and redness at IV infusion sitecommon
  • Dizziness and hot flashes — mild vasomotor symptoms in 3-5% of patientscommon
  • Hyperglycemia in neonates — a signal from one trial (CAP study) showed possibly increased neonatal adverse events, though subsequent meta-analyses have not confirmed thisrare

Frequently Asked Questions

Why is atosiban not approved in the United States?
The FDA declined to approve atosiban in 1998 based on a pivotal trial that failed to demonstrate clear superiority over placebo in a US population, and a signal of possibly worse neonatal outcomes in the atosiban group (though this was likely due to imbalanced randomization with more extremely premature infants in the treatment arm). Subsequent European trials and meta-analyses demonstrated clear efficacy and safety, leading to EU approval in 2000. The manufacturer chose not to pursue a new FDA application, partly because the US market already had other tocolytics and the commercial case was weakened. US obstetricians typically use nifedipine or indomethacin as first-line tocolytics instead.
How does atosiban compare to nifedipine for preterm labor?
Both atosiban and nifedipine are effective tocolytics that delay delivery by 48 hours. The APOSTEL III trial (2014) directly compared them and found similar efficacy in prolonging pregnancy, with no significant difference in neonatal outcomes. Nifedipine has the advantage of oral administration and lower cost. Atosiban has fewer maternal side effects — nifedipine causes headache, flushing, and hypotension more frequently. European guidelines generally allow either as first-line, while favoring atosiban for patients with maternal cardiac disease, diabetes, or hypotension where nifedipine's vasodilatory effects are undesirable.
Can atosiban improve IVF success rates?
Several studies have investigated atosiban administered before and during embryo transfer to reduce uterine contractions that may impair implantation. A meta-analysis of randomized trials showed a trend toward improved clinical pregnancy rates with atosiban, but results remain inconsistent across studies. The rationale is sound — uterine contractions during transfer can expel the embryo — but larger, well-powered trials are needed. Some IVF clinics, particularly in Europe, use atosiban off-label for patients with recurrent implantation failure.
What is the goal of tocolysis in preterm labor?
The primary goal of short-term tocolysis with atosiban or other agents is not to prevent preterm birth indefinitely but to delay delivery by 48 hours. This critical window allows administration of antenatal corticosteroids (betamethasone or dexamethasone), which dramatically reduce neonatal respiratory distress syndrome, intraventricular hemorrhage, and necrotizing enterocolitis if the infant is born preterm. The 48-hour delay also enables maternal transfer to a facility with a neonatal intensive care unit if needed. Long-term maintenance tocolysis has not been shown to improve outcomes and is not recommended.

References

  1. 1
    Atosiban versus beta-agonists for the treatment of preterm labour: a randomised controlled trial(2001)PubMed ↗
  2. 2
    Effectiveness and safety of atosiban versus beta-adrenergic agonists in the treatment of preterm labour: systematic review(2004)PubMed ↗
  3. 3
    Nifedipine versus atosiban in the treatment of threatened preterm labour (APOSTEL III trial)(2013)PubMed ↗

Latest Research

Last updated: 2026-02-19