# Uterine Pharmacology

> Uterine drugs — oxytocin, ergometrine, misoprostol, tocolytics — labour induction, PPH, MBBS Pharmacology notes for obstetrics exams.

- Canonical URL: https://prepelephant.com/topics/mbbs/pharmacology/uterine-pharmacology
- Exam / course: MBBS · Subject: Pharmacology
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "Uterine Pharmacology", PrepElephant, https://prepelephant.com/topics/mbbs/pharmacology/uterine-pharmacology

## Direct answer

Uterine pharmacology splits into drugs that contract the uterus and drugs that relax it. The contractors — oxytocin for induction and postpartum haemorrhage, ergometrine for haemorrhage control, and the prostaglandins (misoprostol, dinoprostone, carboprost) for ripening, termination and refractory bleeding — act on term-pregnant or postpartum myometrium. The relaxers (tocolytics) — nifedipine, atosiban and indomethacin — delay preterm labour for 48 hours to complete steroid lung maturation and in-utero transfer. Every drug in this chapter is judged by one question: does the patient need the baby delivered, the bleeding stopped, or the pregnancy prolonged?

## What you must remember

- Oxytocin is the posterior pituitary hormone; infusion for induction (titrated in milliunits), 5 units IV/IM or 10 units per litre infusion for postpartum haemorrhage; half-life 3–5 minutes so it stops quickly when the infusion stops.
- High-dose prolonged oxytocin has antidiuretic-like action — water intoxication with convulsions is the classical complication.
- Ergometrine 0.2–0.5 mg IM contracts the uterus durably but is contraindicated in hypertension, pre-eclampsia and cardiac disease because it vasoconstricts; never before delivery of the fetus.
- Misoprostol (PGE1) 800 micrograms sublingual is the field treatment for postpartum haemorrhage; it also ripens the cervix and, with mifepristone, terminates pregnancy — hence contraindicated in pregnancy intended to continue.
- Dinoprostone (PGE2) gel or pessary ripens the unfavourable cervix; carboprost (15-methyl PGF2alpha) is the third-line postpartum haemorrhage drug, avoided in asthmatics (bronchoconstriction).
- Tocolysis buys 48 hours: nifedipine (calcium channel blockade) is first-line in most units; atosiban (oxytocin antagonist) is the utero-selective alternative; indomethacin works before 32 weeks but risks premature ductus arteriosus closure and oligohydramnios.
- Ritodrine and other beta-2 tocolytics are largely abandoned — maternal tachyarrhythmia and pulmonary oedema.
- Magnesium sulphate tocolysis is weak; its obstetric role is eclampsia seizure prophylaxis (and fetal neuroprotection under 32 weeks).
- Active management of the third stage — oxytocin 10 units IM at delivery of the anterior shoulder — halves postpartum haemorrhage risk.

## How to work through a postpartum haemorrhage

A woman bleeds heavily five minutes after a vaginal delivery, and the drug sequence is the exam. First, uterine massage and emptying the bladder — an atonic uterus is mechanical before it is pharmacological, and the four Ts (tone, tissue, trauma, thrombin) run the differential. Second, oxytocin 5 units IV slowly (or 10 units IM where no line runs) — fastest onset, safest in hypertension. Third, if the uterus remains boggy, ergometrine 0.25 mg IM — unless pre-eclampsia, hypertension or cardiac disease bars it, and always after the placenta is out. Fourth, misoprostol 800 micrograms sublingual where injections are unavailable — the reason community births in Indian programmes carry misoprostol. Fifth, carboprost 250 micrograms IM every 15 minutes up to eight doses — withheld in asthma. Sixth, tranexamic acid 1 g IV within three hours reduces death from bleeding in the WOMAN trial logic and belongs early, not last. Between the steps, bimanual compression and the operating theatre decision run in parallel — the pharmacology is a ladder, not a menu, and each rung is excluded by one contraindication.

## Where students slip

The classic error is ergometrine (or any uterotonic other than oxytocin) before the fetus is delivered — ergot alkaloids cause sustained tetanic contraction that traps the fetus and ruptures the uterus; oxytocin alone is the inducing agent and even it requires a dilute, titrated infusion. The second slip is misreading indications: misoprostol for a wanted pregnancy in a patient also taking it for NSAID-induced ulcer prophylaxis — the answer is that women of childbearing age on misoprostol must be pregnant-excluded or counseled, a pharmacovigilance favourite. Third, tocolytic selection by gestational age: indomethacin below 32 weeks only, nifedipine first-line generally, atosiban when cardiovascular disease makes beta-2 agonists and nifedipine risky. Finally, remembering that the goal of tocolysis is 48 hours of steroids and transfer, not prevention of preterm birth itself.

## Frequently asked questions

### Why is oxytocin preferred over ergometrine for labour induction?
Oxytocin at low doses produces rhythmic contractions with relaxation between, allowing fetal oxygenation; ergometrine produces tetanic sustained contraction that is fetal-lethal and is reserved for postpartum use.

### Which postpartum haemorrhage drug is contraindicated in asthma and why?
Carboprost, a PGF2alpha analogue, causes bronchoconstriction and can precipitate severe bronchospasm in asthmatics; misoprostol and oxytocin are safe alternatives.

### What is the role of mifepristone and misoprostol in medical termination?
Mifepristone 200 mg orally blocks progesterone receptors and primes the decidua; misoprostol 36–48 hours later completes evacuation — the standard medical abortion regimen up to 63 days in Indian programme practice.

### Why has nifedipine replaced beta-2 agonist tocolytics?
It delays delivery comparably with fewer maternal cardiovascular events — no maternal tachycardia, arrhythmia or pulmonary oedema — and costs less, with monitoring for hypotension and fetal heart rate.

### What is the 48-hour purpose of tocolysis?
To complete a course of antenatal corticosteroids for fetal lung maturation and enable in-utero transfer to a neonatal unit — not to prevent preterm birth, which no tocolytic achieves beyond that window.