According to a drug safety communication(www.fda.gov) from the FDA, administering magnesium sulfate injection to stop preterm labor in pregnant women -- an off-label use of the drug -- for longer than five to seven days may lead to low calcium levels and bone problems in a developing fetus, including osteopenia and fractures.
The agency also noted that the shortest duration of treatment that can result in harm to an infant is not known at this time.
FDA officials identified 18 case reports in the agency's Adverse Event Reporting System that described skeletal abnormalities in newborns exposed in utero to magnesium sulfate. The drug was administered to the infants' mothers for tocolysis during pregnancy. The average duration of in utero exposure was 9.6 weeks, and the estimated average total maternal dose administered was 3,700 grams.
"Magnesium sulfate injection should only be used during pregnancy if clearly needed," the FDA said in the announcement. "If the drug is used during pregnancy, the health care professional should inform the patient of potential harm to the fetus.
"When used in pregnant women for conditions other than its approved indication, magnesium sulfate injection should be administered only by trained obstetrical personnel in a hospital setting with appropriate obstetrical care facilities."
Approved by the FDA to prevent seizures in preeclampsia and to control seizures in eclampsia, magnesium sulfate now will carry new label warnings about the neonatal low calcium levels and bone problems.
The agency said it also is changing the drug's pregnancy category designation from "A" to "D" in light of the positive evidence of human fetal risk coupled with potential benefits, which may make using the drug acceptable in certain situations despite the risks.
Adverse effects that may be related to use of magnesium sulfate can be reported via RxEvent, an adverse drug event reporting service that has partnered with the AAFP, or MedWatch(www.accessdata.fda.gov), the FDA's Safety Information and Adverse Event Reporting Program.