Labor drugs compared — uterotonics, tocolytics and magnesium sulfate
Verified · Oct 2026Uterotonics raise intracellular calcium or tone; tocolytics lower it. Most of them also act on another organ — vessels, airways, gut, heart or the fetus — and that second action is what steers the choice.
Contract or relax: one muscle, opposite drugs
Contracting the uterus: oxytocin acts on a myometrial G protein–coupled receptor, raising intracellular calcium through IP3 so calcium-dependent myosin light-chain kinase drives actomyosin contraction. Methylergonovine, an ergot, acts directly on uterine smooth muscle for a rapid, sustained tetanic contraction. Carboprost is a PGF2α analog whose contractions give hemostasis at the placental site, and dinoprostone (PGE2) ripens the cervix and sensitizes the myometrium to oxytocin.
Relaxing the uterus: terbutaline, a β2 agonist, raises cAMP, which is associated with smooth muscle relaxation; nifedipine inhibits calcium entry into uterine smooth muscle; indomethacin inhibits cyclooxygenase, cutting prostaglandin production. No medication is FDA-approved for tocolysis, and tocolysis aims only at a delay of two to seven days. Magnesium sulfate is labeled to prevent and control eclamptic seizures; its continuous use for preterm labor is unapproved, but given within 24 hours of early preterm birth it provides fetal neuroprotection.
Postpartum hemorrhage: choosing the next drug
Uterine atony is the most common cause of postpartum hemorrhage. WHO recommends IV oxytocin as the first-line uterotonic for treatment, even when oxytocin was given for prevention. When bleeding continues, the second drug is chosen by what else it does:
Methylergonovine constricts vessels, so it is contraindicated in hypertension and toxemia and should be avoided in patients with hypertension; strong and moderate CYP3A4 inhibitors are not coadministered because of ergot vasospasm. Carboprost is labeled for hemorrhage from atony that has not responded to oxytocin, massage and (unless contraindicated) an ergot; because it can cause transient bronchoconstriction it is used cautiously in asthma and avoided in asthmatics, and active cardiac, pulmonary, renal or hepatic disease is a contraindication. Misoprostol has been used for serious postpartum hemorrhage, but that is outside its labeled indication.
Comparison
Labor and delivery drugs at a glance
| Drug | Receptor or mechanism | Main labeled use | Signature hazard | Contraindication that steers the choice |
|---|---|---|---|---|
| Oxytocin | Myometrial GPCR → IP3 → ↑ Ca²⁺ → myosin light-chain kinase | Medically indicated induction or augmentation; postpartum bleeding | Hyperstimulation → uterine rupture, fetal hypoxia; water intoxication | Cephalopelvic disproportion, fetal distress without imminent delivery, hypertonic uterus |
| Methylergonovine | Ergot alkaloid: direct uterine smooth-muscle action, sustained contraction; vasoconstriction | Postpartum atony, hemorrhage and subinvolution after the placenta | Hypertension; vasospasm and myocardial ischemia | Hypertension, toxemia, pregnancy; strong or moderate CYP3A4 inhibitors |
| Carboprost (Hemabate) | PGF2α analog (FP receptor) | Postpartum hemorrhage from atony unresponsive to conventional management | Vomiting and diarrhea; bronchoconstriction; fever | Asthma (caution, avoid); active cardiac, pulmonary, renal or hepatic disease |
| Dinoprostone (Cervidil, Prepidil) | PGE2 (EP receptors); sensitizes myometrium to oxytocin | Cervical ripening at or near term before induction | Uterine tachysystole | IV oxytocics (Cervidil); prior cesarean or uterine surgery |
| Misoprostol (Cytotec) | PGE1 analog (EP2/EP3) | Reducing NSAID-induced gastric ulcers (obstetric use is outside its indication) | Uterine contraction and rupture in pregnancy (boxed); diarrhea | Pregnancy when taken for NSAID ulcers |
| Magnesium sulfate | Competes with calcium at voltage-dependent channels; NMDA-receptor block; neuromuscular block | Prevention and control of seizures in preeclampsia and eclampsia | Loss of reflexes → respiratory depression; fetal bone effects beyond 5–7 days | Myasthenia gravis; heart block or myocardial damage |
| Terbutaline | β2 agonist → adenylyl cyclase → ↑ cAMP | Bronchospasm only (not approved for tocolysis) | Maternal tachycardia, arrhythmia, pulmonary edema, hyperglycemia, hypokalemia | Prolonged (> 48–72 h) or oral tocolysis — boxed |
| Nifedipine | Blocks calcium entry into uterine smooth muscle | Angina (tocolysis is off-label) | Hypotension, flushing, headache | Maternal hypotension or cardiac conditions (great caution) |
| Indomethacin | Cyclooxygenase inhibition → ↓ prostaglandins | Arthritis and gout (tocolysis is off-label) | Premature closure of the fetal ductus arteriosus; oligohydramnios | Pregnancy at about 30 weeks or later (avoid) |
Boxed warnings
| Drug | Boxed warning | The reasoning |
|---|---|---|
| Oxytocin (Pitocin) | Not indicated for elective induction of labor | Available data are inadequate to evaluate its benefits and risks without a medical indication |
| Carboprost (Hemabate) | Strict adherence to recommended dosage, by trained personnel in a hospital with intensive care and surgical facilities | It is a potent oxytocic agent |
| Misoprostol (Cytotec) | Can cause birth defects, abortion, premature birth or uterine rupture in pregnancy | Not taken by pregnant women to reduce NSAID ulcers; rupture risk rises with gestational age and prior uterine surgery |
| Terbutaline | Not for prolonged (injection) or any oral tocolysis, and never maintenance at home | Serious maternal reactions, including death: tachycardia, arrhythmias, pulmonary edema, myocardial ischemia |
| Indomethacin (Indocin) | Cardiovascular thrombotic and gastrointestinal events | The NSAID class warning |
High-yield
Practice questions
Q1A 36-year-old woman who uses an inhaled bronchodilator for asthma has a cesarean delivery. After the placenta is delivered, her uterus stays soft and she keeps bleeding despite an oxytocin infusion. Her blood pressure is 118/72 mm Hg.
Which additional uterotonic is most appropriate?
- A.Methylergonovine
- B.Carboprost
- C.Dinoprostone insert
- D.Terbutaline
Show answer
A. Methylergonovine — Methylergonovine is labeled after delivery of the placenta for uterine atony and hemorrhage, and its contraindications — hypertension, toxemia — do not apply to a normotensive patient. Carboprost can cause transient bronchoconstriction and is avoided in asthmatics. Dinoprostone is labeled for cervical ripening before induction, and terbutaline, a β2 agonist, relaxes rather than contracts smooth muscle.
Q2A 27-year-old woman at 29 weeks with ruptured membranes is in advanced preterm labor and is expected to deliver within 24 hours. She does not have preeclampsia. The team starts magnesium sulfate.
What is the main purpose of the magnesium?
- A.To prolong the pregnancy to term
- B.To provide fetal neuroprotection
- C.To prevent eclamptic seizures
- D.To ripen the cervix
Show answer
B. To provide fetal neuroprotection — Magnesium sulfate given within 24 hours of early preterm birth provides neuroprotection for the neonate, reducing the risk of cerebral palsy. Its continuous use for preterm labor is an unapproved treatment, and tocolysis aims only at a short delay, not term. Eclamptic seizure prevention is its labeled use in preeclampsia, which she does not have, and cervical ripening is the role of dinoprostone.
Q3A student lists four drugs used around delivery, each paired with the reason it is avoided or restricted.
Which pairing is correct?
- A.Methylergonovine — asthma
- B.Oxytocin — postpartum bleeding
- C.Indomethacin — pregnancy at about 30 weeks or later
- D.Terbutaline — tocolysis shorter than 48 hours
Show answer
C. Indomethacin — pregnancy at about 30 weeks or later — NSAIDs, including indomethacin, are avoided at about 30 weeks of gestation and later because they increase the risk of premature closure of the fetal ductus arteriosus. Asthma is a caution for carboprost, not methylergonovine, whose contraindications are hypertension and toxemia. Oxytocin is labeled to control postpartum bleeding, and terbutaline’s box is against prolonged tocolysis beyond 48–72 hours and any oral tocolysis.
Q4A 33-year-old woman taking clarithromycin for pneumonia has postpartum hemorrhage from uterine atony that continues after IV oxytocin. She has no asthma and no cardiac, lung, kidney or liver disease.
Which additional uterotonic is most appropriate?
- A.Carboprost
- B.Methylergonovine
- C.Nifedipine
- D.Misoprostol, because postpartum hemorrhage is its labeled indication
Show answer
A. Carboprost — Strong and moderate CYP3A4 inhibitors such as clarithromycin should not be coadministered with methylergonovine, because ergots with potent CYP3A4 inhibitors have caused vasospasm with cerebral or limb ischemia. Carboprost is labeled for hemorrhage from atony unresponsive to conventional management, and her history has none of its cautions or contraindications. Nifedipine inhibits calcium entry and relaxes the uterus, and misoprostol’s labeled indication is NSAID-induced gastric ulcers.
Sources
- Oxytocin — StatPearls (NCBI)
- Pitocin (oxytocin) — FDA label — FDA / DailyMed
- Uterine Atony — StatPearls (NCBI)
- Postpartum Hemorrhage — StatPearls (NCBI)
- Tocolysis — StatPearls (NCBI)
- WHO consolidated guidelines on postpartum haemorrhage (2025) — World Health Organization
- Methylergonovine maleate injection (American Regent) — FDA label — FDA / DailyMed
- Hemabate (carboprost) — FDA label — FDA / DailyMed
- Cytotec (misoprostol) — FDA label — FDA / DailyMed
- Cervidil (dinoprostone) — FDA label — FDA / DailyMed
- Prepidil (dinoprostone gel) — FDA label — FDA / DailyMed
- Misoprostol — StatPearls (NCBI)
- Prostaglandin E2 (Dinoprostone) — StatPearls (NCBI)
- Prostaglandins — StatPearls (NCBI)
- Eclampsia — StatPearls (NCBI)
- Magnesium Toxicity — StatPearls (NCBI)
- Diagnosis and Treatment of Eclampsia — J Cardiovasc Dev Dis (MDPI) via PMC
- Magnesium sulfate in water for injection (Hospira) — FDA label — FDA / DailyMed
- Magnesium Sulfate — StatPearls (NCBI)
- Magnesium sulfate injection 50% (Fresenius Kabi) — FDA label — FDA / DailyMed
- Preterm Labor — StatPearls (NCBI)
- Terbutaline sulfate injection (Hikma) — FDA label — FDA / DailyMed
- Terbutaline sulfate tablets (Upsher-Smith) — FDA label — FDA / DailyMed
- Nifedipine capsules (Chartwell) — FDA label — FDA / DailyMed
- Indocin (indomethacin) — FDA label — FDA / DailyMed
Each class page compared lists its full sources; this page reuses facts verified there.
By LearnDrugs
Educational summary for medical students. Always verify against current prescribing information and guidelines. Not medical advice.