Updated 2/21/01
Magnesium Sulfate is typically administered through an IV in the hospital. It is not approved by the FDA for use as a preterm labor drug. It is used "off label" for this purpose. It is generally considered to have fewer serious long-term side effects than beta-adrenergics. However, several studies cast doubts about its safety. Magnesium Sulfate it typically administered in a "loading" dose of about 4 or 5 grams and then an hourly dose of 2 to 3 grams. It is given in this high dose to achieve the tocolytic effect. There is a narrow margin between what is considered a "therapeutic" dose of mag sulfate and one that is "toxic." Therefore, women who are using it must be monitored carefully (blood tests to determine magnesium serum levels, reflex checks, etc). In general, a magnesium serum level of about 4 to 6 mEq/L (roughly equivalent to 5 to 7 mg/dl) is considered necessary to quiet uterine activity. "Other investigators have shown that concentrations of 8 to 10 mEq/L result in loss of patellar reflexes with respiratory depression occurring at 10 mEq/L and respiratory arrest at 12 mEq/L or high concentrations." (Cox, Susan, et al., "Randomized Investigation of Magnesium Sulfate for Prevention of Preterm Birth," American Journal of Obstetrics and Gynecology, 1990, Vol 163, p. 767) When combined with beta-adrenergics, Magnesium Sulfate can intensify their cardiovascular side effects. Magnesium Sulfate also is used to prevent seizures in women who have developed pre-eclampsia.
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