Magnesium Sulfate Tocolysis

Abstract & Commentary

By John C. Hobbins, MD, Professor and Chief of Obstetrics, University of Colorado Health Sciences Center, Denver, is Associate Editor for OB/GYN Clinical Alert.

Dr. Hobbins reports no financial relationship to this field of study.

Synopsis: Given its lack of benefit, possible harms, and expense, magnesium sulfate should not be used for tocolysis.

Source: Grimes DA, Nanda K. Magnesium sulfate tocolysis: time to quit. Obstet Gynecol. 2006;108:986-989.

In the vast majority of hospitals magnesium sulfate has been the drug of choice to stop labor in patients presenting with preterm contractions. The usual regimen of the loading dose of 4-6 grams followed by 1.5-5 grams per hour has been so well entrenched in obstetrical care that the magnesium bottle is hung the minute the preterm labor patient signs her hospital admission papers. Yet, nobody has questioned this practice until recently, when David Grimes, a noted authority on evidence based medicine, decided to take on magnesium sulfate.

In an editorial in Obstetrics & Gynecology, Grimes reviewed the literature comparing magnesium sulfate with placebo or other tocolytics in the treatment of preterm labor. He particularly focused on the Cochrane database which included 2000 patients in 23 studies. Magnesium sulfate had no advantage over controls with regard to delivery within 48 hours, delivery before 34 weeks, or delivery prior to 37 weeks. If this was a surprise to some, the "do no harm" part was equally as attention getting. Seven studies involving 727 patients found that the relative risk of pediatric death with magnesium sulfate was 2.8 (95% CI, 1.2-6.6). Also, its usage was not associated with the decrease in reparatory distress syndrome, interventricular hemorrhage, or necrotizing enterocolitis.

Maternal mortality was increased 4.7 fold when the above standard dose was utilized for more than 24 hours (total dose of 48 grams).


The title of the above editorial was "Magnesium sulfate tocolysis: time to quit." This may be tough to do, especially when seemingly the alternative is to do nothing. Although going "cold turkey" is very unappealing, our colleagues in hospital administration will be all too happy to shave our hospital staff by at least one nursing FTE for all the IV infusions that will not need to be started and maintained.

Dr. Grimes does offer an alternative treatment, a calcium channel blocker—nifedipine. A Cochrane database indicates a reduction in births within 7 days of therapy (relative risk, 0.76; 95% CI, 0.60-0.97) and at less then 34 weeks (relative risk, 0.83; 95% CI, 0.69-0.99) when nifedipine is used for preterm labor. Certainly the therapy is far better tolerated than magnesium sulfate. Unfortunately, the editorial did not touch upon nifedipine's ability to stop labor for 48 hours—long enough to get steroids on board. This is what we thought we were accomplishing with magnesium sulfate.