A drug commonly used to arrest preterm labor, magnesium sulfate, works just as well as another commonly used drug, nifedipine, but is much more likely cause mild to serious side effects in pregnant women, a study shows.

Magnesium sulfate is a tocolytic, meaning it inhibits uterine contractions. Dr. Deirdre J. Lyell from Stanford University Medical Center, and colleagues compared the efficacy and side effects of magnesium sulfate and nifedipine in 192 pregnant women in preterm labor. Ninety-two women were treated with magnesium and 100 with nifedipine.

Results showed that preterm delivery was delayed by 48 hours more often with magnesium (87 percent versus 72 percent), but it took less time for uterine contractions to subside with nifedipine (6.1 versus 8.4 hours).

Overall, there were no differences between the two drugs in terms of the proportion of women who delivered within 48 hours, gestational age at delivery, episodes of recurrent preterm labor, or major neonatal outcomes, the authors report.

However, maternal side effects were twice as likely with magnesium sulfate as with nifedipine (65 percent versus 34 percent).

Serious adverse effects - shortness of breath, fluid build-up in the lungs, low blood pressure, and chest pain - occurred in 22 percent of women in the magnesium group and 10 percent in the nifedipine group. Lethargy, nausea and vomiting, and dizziness were also significantly more common with magnesium.

“There is no free lunch with any of these drugs,” Lyell said in a press release. “Magnesium sulfate has some particularly unpleasant side effects, (whereas) nifedipine often leaves women feeling better.”

SOURCE: Obstetrics and Gynecology, July 2007.