The FDA is changing the label of magnesium sulfate to warn against its use for more than 5 to 7 days to stop preterm labor. Long-term use can lead to hypocalcemia, osteopenia, and fractures in the infant. In addition, the drug’s pregnancy category is being changed from A to D, indicating “positive evidence of human fetal risk.”
The FDA reemphasizes that magnesium sulfate is not approved to stop preterm labor. It is unknown whether a shorter treatment duration would result in harm to the fetus.
The warning comes after the FDA reviewed epidemiologic data and 18 cases of skeletal abnormalities in infants exposed to magnesium sulfate in utero (average duration of exposure, 10 weeks). The epidemiologic data indicate that laboratory results normalize within days of birth, but long-term clinical effects on bone health aren’t known.
Asked to comment, Allison Bryant of Journal Watch Women’s Healthwrote: “The use of magnesium sulfate as a tocolytic persists in many institutions. In addition, data suggesting its effectiveness in providing neuroprotection for fetuses exposed in utero and born at early gestational ages have led to a resurgence in use. However, there are no data suggesting value to using magnesium sulfate for tocolysis for longer than 48 hours, for neuroprotection beyond when delivery is felt to be imminent, or for seizure prophylaxis in the setting of preeclampsia beyond 24 hours postpartum. Therefore, this FDA warning is unlikely to greatly alter current clinical practice.”