Pharmacology · Ergot Alkaloid Pharmacology

Obstetric Ergot Alkaloids

Methylergonovine and ergometrine — mechanisms, clinical use, and contraindications

The obstetric ergot alkaloids are a clinical application built directly on the same serotonin 5-HT2A and alpha-adrenergic receptor activity that makes the vasoconstriction of Module 1 dangerous. Here that vasoconstriction is directed at uterine smooth muscle, where it is the intended therapeutic effect rather than a side effect to be avoided.
Postpartum hemorrhage is the leading cause of maternal death globally. The uterotonic drugs — oxytocin, methylergonovine, ergometrine, carboprost — are the pharmacological cornerstone of prevention and treatment. Knowing which drug to use and which to avoid in any given patient requires understanding both the shared mechanism and the differences in receptor selectivity and adverse effect profile.
The comparison card places oxytocin and methylergonovine side by side because clinical decisions frequently require choosing between them or combining them. Oxytocin acts on its own receptor with a shorter duration; methylergonovine acts through serotonin and alpha-adrenergic receptors with a longer duration but adds the cardiovascular liability that makes hypertension its defining contraindication.
The contraindication card is the single most important clinical safety point in this module. Methylergonovine and ergometrine cause sustained peripheral and uterine vasoconstriction that includes the coronary arteries. In a patient with hypertension or known cardiovascular disease, that vasoconstriction can precipitate a hypertensive crisis or coronary vasospasm. Pre-eclampsia and eclampsia are absolute contraindications.
The drug interaction rule box closes the module because the cytochrome P450 3A4 interactions that appeared in Module 1 apply with equal or greater force here — methylergonovine plasma levels can increase enough with azole antifungals or macrolides to cause symptomatic vasospasm even at standard obstetric doses.

Suggested References

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