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STANFORD, Calif., July 2 -- Compared with nifedipine (Procardia), magnesium sulfate was significantly more likely to cause serious maternal side effects when used to prevent preterm labor.

These side effects include dyspnea, chest pain, and pulmonary edema, reported Deirdre L. Lyell, M.D., of Lucile Packard Children's Hospital and Stanford, and colleagues, in the July issue of Obstetrics & Gynecology.

However, magnesium sulfate, which is the drug most commonly used to prevent preterm labor, was more likely to succeed in that objective, according to results of a randomized controlled trial.

Babies born to mothers taking magnesium sulfate were more likely to be admitted to the neonatal intensive care unit (P=0.04) and had significantly longer stays in the NICU than babies born to mothers in the nifedipine arm (P=0.007).

Nonetheless, there were no significant differences in delay of delivery, gestational age at delivery, and neonatal outcomes, said Dr. Lyell.

And, 87% of the women treated with magnesium sulfate delayed labor for at least 48 hours versus 72% of women in the nifedipine arm, a small but statistically significant difference (P=0.01).

However, given the significantly higher rate of maternal adverse effects, the researchers said physicians should give more weight to those potential adverse effects when selected a drug for acute tocolysis of preterm labor.

In addition to the serious side effects, women randomized to magnesium sulfate were also significantly more likely to have nausea, vomiting, lethargy, flushing, and blurry vision, than women treated with nifedipine (P<0.001 for all).

"There is no free lunch with any of these drugs," Dr. Lyell said. "But magnesium sulfate has some particularly unpleasant side effects, including vomiting, lethargy and blurry vision. The alternative treatment, nifedipine, often leaves women feeling better."

The trial enrolled 192 women who were in active preterm labor at 24 to 33 weeks and six days of gestation. The primary outcome was arrest of preterm labor, defined as prevention of delivery for at least 48 hours.

Ninety-two women were randomized to magnesium sulfate. The average age of women was 27 and the average gestational age at enrollment was 31 weeks. Thirty-three women in the magnesium sulfate group required maintenance tocolysis and four women in each arm had premature rupture of membranes.

Among the findings:

• Sixty-five percent of women in the magnesium sulfate arm had one or more treatment-related adverse effects versus 34% of women in the nifedipine arm (P<0.001).
• Twenty-two percent of women in the magnesium sulfate arm had serious adverse effects including shortness of breath, pulmonary edema, hypotension, and chest pain versus 10% of women who received nifedipine (P=0.03).
• Fifty-two percent of babies born to women treated with magnesium sulfate were admitted to the NICU, versus 37% of babies whose mothers were given nifedipine to stop labor (P=0.04).
• Among babies admitted to the NICU, those whose mothers received magnesium sulfate stayed an average of 8.8 days versus 4.2 day average NICU stays for babies born to mothers in the nifedipine group (P=0.007).

Dr. Lyell concluded that in her experience women given magnesium sulfate remember it and it is not a pleasant memory.

Although magnesium sulfate is a clinically appropriate treatment, she noted that tocolytic agents are "prescribed under what are already very difficult circumstances for the patients, and side effects are very important to them."

Dr. Lyell reported no potential conflicts of interest. The funding source for the study was not disclosed.

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