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Atosiban (Tractocile)9 residues (approx.)CYITNCPRGEach bubble = one amino acid. Size = residue mass. Color = chemical class.Uses closest standard amino acids for non-standard residues.Also known as: Tractocile, RWJ-22164, ORG 5436
Six randomized controlled trials back its ability to delay preterm delivery by at least 48 hours. Atosiban (brand name Tractocile) is an oxytocin receptor antagonist given by IV in a hospital to slow contractions long enough for corticosteroids to reach fetal lungs. The EMA approved it in January 2000; the FDA never did. APOSTEL 8 (Lancet, 2025, n=752) found no improvement in composite neonatal outcomes versus placebo at 30 to 33 weeks (RR 0.90, 95% CI 0.58 to 1.40). Obstetricians in Europe and parts of Asia still reach for it when beta-agonist tocolytics carry too much cardiovascular risk, especially in twin and triplet pregnancies.
The APOSTEL 8 trial (Lancet, 2025, n=752) changed the conversation around this drug. The study found no improvement in composite neonatal outcomes compared to placebo at 30 to 33+6 weeks of gestation (RR 0.90, 95% CI 0.58 to 1.40). That result puts a hard question mark on routine use beyond 30 weeks. Atosiban (Tractocile, CAS 90779-69-4) is a synthetic nonapeptide that competitively blocks both oxytocin and vasopressin V1a receptors on uterine muscle cells. The EMA approved it in January 2000 for acute tocolysis between 24 and 33+6 weeks of gestation. The mechanism is specific. By blocking Gq/11-coupled signaling at the oxytocin receptor, atosiban prevents the intracellular calcium release that drives myometrial contraction. Contraction frequency drops within 10 minutes of the IV bolus. That speed matters because the entire clinical rationale is to buy a 48-hour window for betamethasone to mature fetal lungs. Maternal tolerability is its strongest clinical argument. Six RCTs showed less tachycardia, less tremor, and less hypotension compared to beta-agonist tocolytics like ritodrine [1]. Placental transfer is minimal, with a fetal-to-maternal plasma ratio of 0.12 [3]. The drug sits in a complicated place right now. Approved, widely used in European labor wards, but 2025 data suggests the delivery delay it reliably achieves may not translate into better outcomes for the baby at later gestational ages. The EMA label has not been updated as of April 2026.
Atosiban works through competitive antagonism at two receptor targets on uterine myometrial cells: the oxytocin receptor (OTR) and the vasopressin V1a receptor. Blocking the OTR is the primary action. Under normal conditions, oxytocin binds the OTR and activates Gq/11 signaling, which triggers phospholipase C. That enzyme cleaves membrane lipids into IP3 and DAG. IP3 releases calcium from intracellular stores; DAG activates protein kinase C. Both pathways converge on the actin-myosin machinery that makes uterine smooth muscle contract. Atosiban sits on the OTR and prevents this entire cascade. It also blocks oxytocin-mediated release of prostaglandins PGE2 and PGF2alpha from decidual tissue. Those prostaglandins independently stimulate contractions, so shutting them down adds a second layer of suppression. The V1a receptor component matters more than it might seem. V1a receptors are upregulated in myometrial tissue during late pregnancy. They drive contractile tone independently of oxytocin signaling. Blocking both receptors gives atosiban broader tocolytic coverage than a pure OTR antagonist would achieve. At recommended IV doses, steady-state plasma levels reach approximately 442 ng/mL within one hour. The drug clears quickly after infusion stops, with a terminal half-life of about 1.7 hours and clearance of 41.8 L/h.
EMA-approved tocolytic (Tractocile, 2000) that delays delivery ≥48h vs placebo. APOSTEL 8 (Lancet, 2025, n=752) found NO superiority over placebo in composite neonatal outcomes at 30–33+6 weeks (RR 0.90, 95% CI 0.58–1.40), questioning routine use. Delivery delay benefit confirmed; neonatal benefit remains unproven.
APOSTEL 8 (Lancet, 2025, PMID 40049187, n=752); APOSTEL III (Lancet, 2016, PMID 26944026, n=510)
APOSTEL 8 (2025): no neonatal outcome benefit vs placebo at 30–33+6 weeks. FDA rejected NDA (2000): no US approval. Off-label IVF/FET use: 2025 RCT (n=1,100, PMID 40036869) showed no significant live birth improvement (49.5% vs 44.7%, p=0.10). All evidence is in hospital setting only: no community pharmacokinetic or efficacy data.
No community use exists. Atosiban is an IV-only drug administered exclusively in a hospital obstetric unit. There are no consumer protocols, self-administration reports, or Reddit/forum discussions. All "community" context is drawn from clinical literature and patient experience within the hospital setting.
No community use exists by definition (hospital-only IV drug). Within the scientific literature, APOSTEL 8 (Lancet, 2025) introduces significant internal scientific divergence: EMA approval (2000) was granted on the basis of delivery delay vs placebo, but the 2025 RCT shows no improvement in the neonatal outcomes that are the actual goal of tocolysis. The label has not been updated as of April 2026.
| Level | Dose / Injection | Frequency |
|---|---|---|
| Beginner | 6.75mg | Single dose |
| Moderate | 6.75mg | Single dose |
| Aggressive | 6.75mg | Single dose |
Atosiban is a hospital-only IV drug. There is no subcutaneous version, no self-injection protocol, no home use. Your obstetric team manages everything. The three-phase protocol is fixed: 6.75 mg bolus over 1 minute, then 300 mcg/min infusion for 3 hours, then 100 mcg/min for up to 45 hours. Maximum total dose per course is 330.75 mg. Treatment cannot exceed 48 hours. Two separate vial types exist and they are not interchangeable. The bolus vial is 0.9 mL containing 6.75 mg (7.5 mg/mL concentration). The infusion concentrate vial is 5 mL containing 37.5 mg (7.5 mg/mL). For Phase 2 and 3, dilute each 5 mL concentrate vial into 45 mL of 0.9% sodium chloride, Ringer's lactate, or 5% dextrose. Final concentration: 0.75 mg/mL. Loading rate: 24 mL/hour. Maintenance rate: 8 mL/hour. If contractions return after stopping, retreatment with the identical protocol is permitted. Up to 3 courses are documented clinically. One thing most non-specialists miss: after the infusion ends, residual OTR blockade can reduce uterine tone at delivery. Make sure delivery notes flag prior atosiban use so uterotonic agents are standing by.
Atosiban is not cycled. It is given as a single acute course of up to 48 hours in a hospital setting for preterm labor management. Re-treatment may be initiated if contractions recur, using the same bolus + infusion protocol.
Atosiban is not a cycled compound. It is administered as a single acute course of up to 48 hours per episode of threatened preterm labor. Re-treatment using the identical three-phase protocol is initiated only when contractions recur, with no required off-period between courses: timing is clinically driven, not schedule-driven. Up to 3 courses are documented in the clinical literature.
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Expected: Significant reduction in uterine contractions within 10 minutes. Delivery delay ≥48h confirmed. APOSTEL 8 (2025): no significant improvement in composite neonatal outcomes vs placebo at 30–33+6 weeks (RR 0.90, 95% CI 0.58–1.40).
Monitor: Continuous fetal heart rate monitoring (CTG) and tocometry throughout. Maternal BP and pulse every 15–30 min during loading phase; hourly during maintenance. Hourly respiratory assessment in multiple gestations or when concomitant tocolytics previously used. Signs of chorioamnionitis every 4–6h. Blood glucose at baseline and every 12h in gestational diabetes.
Screen for chorioamnionitis (temperature, uterine tenderness, discharge character, WBC). Rule out contraindications including premature rupture of membranes after 30 weeks, fetal distress, placenta previa, and eclampsia.
Administer the IV bolus: draw up the entire 0.9 mL bolus vial (6.75 mg). Inject slowly over exactly 1 minute into a running IV line. Do not push faster.
Dilute 37.5 mg/5 mL concentrate vials into 0.9% NaCl, Ringer's lactate, or 5% dextrose to achieve 0.75 mg/mL. Set infusion pump to 24 mL/hour (300 mcg/min). Run for 3 hours. Total loading dose: 54 mg.
At the 3-hour mark, reduce to maintenance infusion at 8 mL/hour (100 mcg/min). Continue for up to 45 hours. Maximum maintenance dose: 270 mg.
Monitor continuously: fetal heart rate by CTG, uterine contraction frequency by tocometry, maternal BP and pulse every 15 to 30 minutes during loading then hourly during maintenance. Respiratory assessment hourly; increase to continuous in multiple gestations.
Stop infusion at 48 hours maximum, or sooner if: fetal distress occurs, chorioamnionitis is confirmed, delivery becomes indicated, or pulmonary symptoms develop.
Total maximum dose per course: 330.75 mg.
Standard concurrent therapy: the 48-hour tocolytic window exists specifically to allow two doses of betamethasone (12 mg IM × 2, 24h apart) to accelerate fetal lung maturity. This combination is the entire clinical purpose of atosiban.
Betamethasone 12 mg IM, repeat at 24h; begin within first 48h of atosiban infusion
Used concurrently for fetal neuroprotection (reduces cerebral palsy risk in preterm neonates <32 weeks). May be co-administered during atosiban infusion per institutional protocol.
Per protocol; typically 4 g IV loading then 1 g/hr maintenance ≤32 weeks
Direct pharmacological antagonism: atosiban is a competitive OT/V1a receptor antagonist. Concurrent oxytocin directly opposes the tocolytic mechanism and negates efficacy.
Do not combineCarbetocin is a long-acting oxytocin receptor agonist: directly opposes atosiban's OTR blockade at the same receptor.
Do not combineConcurrent use significantly increases risk of pulmonary edema, particularly in multiple gestations. All documented post-marketing pulmonary edema cases occurred ~40h into infusion and involved combination tocolytic use.
Do not combineAdditive pulmonary edema risk when used with atosiban, especially in multiple pregnancies or nulliparous women. Do not overlap if switching tocolytics.
Pricing updated 2026-04-09
The most serious post-marketing signal is pulmonary edema. It is rare, but every documented case shares the same pattern: onset around 40 hours into infusion, in patients carrying twins or triplets, and nearly always with concomitant tocolytic use. That specificity makes risk prediction straightforward but the consequence severe. Continuous respiratory monitoring (SpO2, respiratory rate, auscultation) is standard during any atosiban infusion lasting beyond 24 hours. Nausea is the most common maternal side effect at 14% per the EMA SmPC. It typically appears early, within the first hour after bolus injection, and is usually self-limiting. Common reactions (1 to 10%) include headache, dizziness, tachycardia, hypotension, hot flushes, and vomiting. These are substantially less frequent and less severe than the cardiovascular effects seen with beta-agonist tocolytics. The comparison is part of why European guidelines favor atosiban. Uncommon effects (0.1 to 1%) include pruritus, rash, uterine hemorrhage, injection site reactions, pyrexia, and hyperglycemia. The hyperglycemia signal deserves attention in patients with gestational diabetes; blood glucose monitoring every 12 hours is recommended in that population. Rare effects include allergic reactions, insomnia, and uterine atony. The atony concern extends into the postpartum period: residual oxytocin receptor blockade can reduce uterine tone after delivery. Uterotonic agents (oxytocin infusion, ergometrine) should be available at clinical handover. One safety signal towers above all others in this drug's history. In the important trials, 11 of 15 fetal and infant deaths in the atosiban arm occurred in patients at less than 24 weeks of gestation. This finding is why 24 completed weeks is the absolute gestational age floor for use. Below that threshold, fetal mortality was not comparable to placebo. This contraindication is backed by trial-level mortality data. Pregnancy status is not a "warning" here because pregnancy is the entire use case. But the gestational window matters enormously: only 24+0 to 33+6 weeks. Outside that range, risk-benefit reverses. Any patient developing dyspnea, cough, or oxygen desaturation during infusion should have atosiban stopped immediately and receive a chest assessment. Do not wait for a formal pulmonary edema diagnosis before stopping.
Verify Atosiban (Tractocile) dosing and safety with a second opinion
Hospital-formulary pharmaceutical product administered under direct obstetric supervision. EU brand: Tractocile by Ferring Pharmaceuticals. Indian generics: Atoziban (Sun Pharma), Tosiban (Zuventus). No compounded, gray-market, or consumer supply chain exists. All use is through regulated hospital pharmacy.
| Test | When | Target |
|---|---|---|
| Fetal heart rate (CTG / electronic fetal monitoring) | Continuously throughout infusion | Normal FHR 110–160 bpm with adequate variability; no repetitive late or variable decelerations |
| Uterine contraction frequency and intensity (tocometry) | Continuously throughout infusion | <4 contractions per 30 minutes; reducing trend from pre-treatment baseline |
| Maternal pulse and blood pressure | Every 15–30 min during loading phase (Phase 2); hourly during maintenance (Phase 3) | BP >90/60 mmHg; HR <100 bpm |
| Respiratory assessment (SpO₂, respiratory rate, auscultation) | Hourly; immediately on any dyspnea, cough, or SpO₂ decline | SpO₂ >95%; RR 12–20/min; clear lung fields on auscultation |
| Chorioamnionitis screen (maternal temperature, uterine tenderness, vaginal discharge character, WBC) | At baseline before initiation; every 4–6h during infusion | Temperature <38.0°C; no uterine tenderness; no purulent discharge; WBC within gestational norm |
| Blood glucose | At baseline; every 12h in patients with gestational diabetes or pre-existing diabetes | Fasting <5.3 mmol/L (95 mg/dL); 2h post-meal <6.7 mmol/L (120 mg/dL) per GDM targets |
Detect fetal distress requiring immediate delivery despite ongoing tocolysis
Assess tocolytic response; determine if retreatment or additional intervention is needed
Detect tachycardia and hypotension (common ADRs, 1–10% per SmPC)
Early detection of pulmonary edema (post-marketing signal; peak risk ~40h; multiple gestations at highest risk)
Chorioamnionitis is an absolute contraindication: tocolysis during intrauterine infection delays delivery while infection worsens, risking maternal sepsis and fetal death
Hyperglycemia is an uncommon (0.1–1%) ADR per EMA SmPC; clinically significant in GDM patients
Rapid onset of tocolytic effect after IV bolus; significant reduction in uterine contractions.
Steady-state plasma concentration achieved (~442 ng/mL); stable uterine quiescence.
Transition from high-dose loading infusion (300 mcg/min) to maintenance infusion (100 mcg/min).
Critical treatment window: corticosteroids administered to mature fetal lungs; majority of patients remain undelivered.
Rapid decline in plasma levels after stopping infusion (terminal half-life ~1.7 hours). Uterine activity may return.
1 to 10 minutes: Contraction frequency and intensity drop quickly after the IV bolus hits. OTR occupancy reaches therapeutic levels within minutes. Nausea is the most common early reaction at 14% per the EMA label. Flushing and headache also appear during this window. 1 hour: Plasma concentration settles at steady-state, roughly 442 ng/mL. Uterine quiescence stabilizes. Tachycardia, hypotension, and dizziness can appear at this stage (1 to 10% frequency). Injection site reactions may become noticeable. 3 hours: The infusion rate drops from 300 mcg/min to 100 mcg/min. Uterine suppression holds at the lower rate. This is the window where the first betamethasone dose typically goes in. 24 to 48 hours: The core treatment window. Betamethasone has time to act on fetal lungs. Most patients remain undelivered at 48 hours versus placebo. But APOSTEL 8 (Lancet, 2025, n=752) found no improvement in composite neonatal outcomes versus placebo at 30 to 33+6 weeks. Pulmonary edema risk peaks around the 40-hour mark, particularly in multiple gestations with prior tocolytic use. Hyperglycemia (0.1 to 1%) warrants monitoring in gestational diabetes patients. Post-infusion: Atosiban clears rapidly once the infusion stops. Terminal half-life is about 1.7 hours; clearance runs at 41.8 L/h. Contractions typically return within hours. Retreatment with the same protocol is an option if they do. Residual OTR blockade can reduce uterine tone at delivery. Uterotonic agents should be staged for the handover.
Significant reduction in contraction frequency and intensity within 10 minutes of IV bolus; rapid OTR occupancy at therapeutic plasma levels.
N/A: hospital setting only
Steady-state ~442 ng/mL achieved within 1 hour of infusion start; stable uterine quiescence; maternal cardiovascular parameters should be stabilizing.
N/A
Rate reduced from 300 mcg/min to 100 mcg/min; uterine suppression maintained at lower infusion rate. Window for betamethasone first dose.
N/A
Primary tocolytic goal: 48h for betamethasone to act on fetal lungs and for fetal transfer if required. Majority undelivered at 48h vs placebo. APOSTEL 8 (2025): NO significant improvement in composite neonatal outcomes vs placebo at 30–33+6 weeks.
N/A
Rapid plasma clearance (terminal t½ ~1.7h; clearance 41.8 L/h). Uterine activity typically returns within hours of stopping. Retreatment may be initiated on recurrence.
N/A
Source: Terminal half-life (tβ) from EMA SmPC; initial half-life (tα) 0.21 hours. Clearance 41.8 L/h, Vd 18.3 L.
Loading the interactive decay curve.
Atosiban (Tractocile) holds EMA marketing authorization since January 2000 for tocolysis between 24+0 and 33+6 weeks of gestation. It is available through hospital formularies across EU member states, the UK, and several countries in Asia and Latin America. The FDA rejected the US marketing application in 2000. The rejection was based on a domestic trial showing no neonatal benefit versus placebo, combined with the mortality signal in the sub-24-week subgroup. As of April 2026, atosiban remains unavailable in the United States. US clinicians use nifedipine or indomethacin as alternative tocolytics. Generic atosiban is available in India through manufacturers including Sun Pharma (brand: Atoziban) and Zuventus (brand: Tosiban). These require GMP certification verification before institutional use. Atosiban is not a WADA-listed substance and has no relevance to athletic testing. It has no consumer or retail distribution channel anywhere in the world. All supply flows through hospital pharmacy procurement. This content is for informational purposes only and does not constitute medical advice. Atosiban is a prescription-only hospital medication administered under obstetric supervision.
Peptide Schedule Research TeamReviewed Apr 20269 Citations
