OB/ PHARM | Methergine (WARNING)

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Methergine (methylergonovine) is an oral and parenteral semisynthetic ergot alkaloid derivative classified as a labor inducer. It is primarily indicated for the management and prevention of postpartum uterine atony, subinvolution of the uterus, and postpartum hemorrhage following the delivery of the placenta or the anterior shoulder.

Mechanism of Action The drug acts directly on uterine smooth muscle to increase the tone, rate, amplitude, strength, duration, and frequency of rhythmic contractions. This rapid and sustained tetanic uterotonic effect reduces blood loss and shortens the third stage of labor. Pharmacologically, methylergonovine is a selective antagonist of serotonin receptors in various smooth muscles, a partial agonist of serotonin in umbilical and placental blood vessels, and a partial agonist of alpha-adrenergic receptors in blood vessels. This makes it a potent vasoconstrictor, which can cause coronary vasospasm and a decrease in luminal diameter, particularly after parenteral administration.

Handling and Safety Classified as a hazardous drug on the NIOSH Group 3 and Draft 2020 Table 2 lists, Methergine requires strict handling precautions. Healthcare workers must use double chemotherapy gloves and protective gowns when handling injectable or oral liquid forms. Eye, face, and respiratory protection may also be required, and manipulating or crushing oral tablets necessitates additional protective equipment to prevent exposure.

Administration and Dosage The drug has a duration of action of approximately three hours across all administration routes:

• Oral: Administered as 0.2 mg 3 to 4 times daily for up to 7 days postpartum. It has roughly a 60% bioavailability, with a slower absorption rate in postpartum females compared to healthy adult males. The onset of action occurs in 5 to 10 minutes.
• Intramuscular (IM): This is the preferred parenteral route due to a lower incidence of side effects. A 0.2 mg dose is injected deeply into a large muscle, with an onset of 2 to 5 minutes. Doses can be repeated every 2 to 4 hours as needed.
• Intravenous (IV): IV administration is not routinely recommended and should only be used as a strict life-saving measure due to the severe risk of inducing sudden hypertensive and cerebrovascular accidents. It must be given slowly over a period of no less than 60 seconds. Onset is immediate, resulting in a strong oxytocic, cardiovascular, and cerebrovascular response. Periarterial or intraarterial injection must be strictly avoided. Parenteral solutions must be visually inspected and discarded if they are discolored or not clear.

Monitoring and Adverse Reactions Due to its strong vasoconstrictive properties, providers must closely monitor the patient's blood pressure, heart rate, and uterine response prior to and during administration. Adverse reactions include:

• Severe: Stroke, myocardial infarction, ventricular fibrillation, seizures, uterine rupture, water intoxication, and anaphylactoid reactions.

Pharmacokinetics and Drug Interactions Methylergonovine is highly unstable when exposed to tropical conditions of heat, light, and moisture. It has a short elimination half-life, is rapidly eliminated, and is extensively metabolized by the liver, with less than 5% of an oral dose excreted in urine. No drug accumulation occurs with repeated oral dosing. Importantly, the use of CYP3A4 inhibitors can increase the risk of severe ergot toxicity,