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Obstetric Ergot Alkaloids
Module 2 — Methylergonovine: Mechanism, Use, and Contraindications
Mechanism of Action
Receptor Targets
Dual Receptor Mechanism
  • Alpha-adrenergic receptor activation
  • Serotonin type 2 receptor activation
  • Both increase intracellular calcium
  • Result: sustained tetanic uterine contraction
vs. Oxytocin
Contraction Character
  • Oxytocin: rhythmic, wave-like contractions
  • Methylergonovine: sustained, tetanic contraction
  • Methylergonovine compresses uterine vessels more continuously
  • Oxytocin: first-line; methylergonovine: second-line
Clinical Use
Indication
Postpartum Hemorrhage
  • Prevention and treatment of uterine atony
  • Given after placental delivery — never before
  • Intramuscular route: preferred, onset 2–5 minutes
  • Oral: maintenance only, not acute hemorrhage
Contraindications
When Not to Use
  • Hypertension — absolute contraindication
  • Preeclampsia or eclampsia — absolute contraindication
  • Coronary artery disease or peripheral vascular disease
  • Before placental delivery — risk of entrapment
Adverse Effects and Drug Interaction
Key Points
Adverse Effects and Cytochrome P450 3A4 Interaction
  • Hypertension — monitor blood pressure after administration
  • Nausea and vomiting — dopaminergic mechanism
  • Erythromycin and clarithromycin: contraindicated — cytochrome P450 3A4 inhibition raises drug levels
  • Azithromycin: safe alternative macrolide (does not inhibit cytochrome P450 3A4)
Step 1 Rule
Methylergonovine is contraindicated in hypertension and preeclampsia. It is given after placental delivery only. Macrolide antibiotics that inhibit cytochrome P450 3A4 (erythromycin, clarithromycin) are contraindicated with this drug. Use azithromycin when a macrolide is needed.