Magnesium Sulfate

Magnesium sulfate is used to treat pre-eclampsia, eclampsia and preterm labor.

Pre-eclampsia (also known as toxemia and Pregnancy-Induced High Blood Pressure) consists of high blood pressure, protein in the urine and edema (swelling). It can rapidly become severe pre-eclampsia, with very high blood pressure, visual disturbances, failing kidneys and elevated liver enzymes. In rare cases, pre-eclampsia develops into eclampsia, where potentially fatal convulsions occur. It also can become HELLP Syndrome (hemolysis (H), which is the breaking down of red blood cells, elevated liver enzymes (EL), and low platelet count (LP)), which is potentially fatal to both the woman and her baby or babies.

Treating Pre-Eclampsia and Eclampsia
Questions to Discuss with Your Doctor

Preterm labor is defined as contractions and cervical effacement and/or dilation before term.

Treating Preterm Labor
Table and Study Citations for Use in Preterm Labor
Questions to Discuss with Your Doctor

Treating Pre-Eclampsia and Eclampsia

1. Why is magnesium sulfate used for pre-eclampsia and eclampsia?

This drug typically is used in obstetrics to prevent severe pre-eclampsia from becoming eclampsia (life-threatening convulsions). It also is used to stop the convulsions of eclampsia. In the United States, it has been used to treat severe pre-eclampsia for 60 years (Lancet, 1997, Vol. 350, p. 1491) and is FDA-approved for this purpose.

2. How is it administered?

It usually is given to patients through an IV in the hospital, and sometimes it is administered in shots.

A woman experiencing pre-eclampsia may receive a loading (big) dose through an IV of 4 to 6 grams and a continuous dose of 1-2 grams per hour. She may receive these doses before giving birth and sometimes for at least 24 hours after giving birth. If a woman is having convulsions from eclampsia, she may receive a single dose of 4 to 6 grams to try to stop the convulsions.

3. How long do patients take magnesium sulfate?

In severe pre-eclampsia, magnesium sulfate is used for short periods of time (24 to 48 hours) until the baby or babies can be delivered, which is the only "cure" for pre-eclampsia. If the baby or babies will be premature, it can buy enough time to administer drugs to the woman to strengthen the baby's lungs (corticosteroids like betamethasone).

4. a. What are some of the most common side effects of magnesium sulfate?

4. b. What are some of the complications of magnesium sulfate(sometimes occurring with magnesium overdose)?
Note: Your doctor can ensure your safety by monitoring you carefully and by making sure your kidneys are fully functioning. This can be done with a blood test. Severe pre-eclampsia sometimes can cause a woman's kidneys to fail, and that can intensify the risk of a magnesium sulfate overdose. A blood test can check the level of magnesium in your body.

Rare, Severe Complications

Adapted from: Hill, Washington Clark, "Risks and Complications of Tocolysis," Clinical Obstetrics and Gynecology, 1995, Vol. 38, p. 733.

5. Does magnesium sulfate affect my baby or babies?

Yes, it crosses the placenta. After your baby or babies are born, they may have some of it in their blood. Magnesium levels usually return to normal within a few days.

Click here for a list of neonatal side effects.

In the past, magnesium sulfate has been thought to be safe for babies whose mothers take it. However, doctors are debating the significance of one 1997 study that challenges this view. More studies are needed before definite conclusions can be made. In addition, because of the significant risk of developing eclampsia, the benefits of a woman being on magnesium sulfate may outweigh the risks to her and to her baby.

This 1997 study was conducted at the University of Chicago and found an increased number of neonatal deaths in women who took it for preterm labor in comparison to those women who took another preterm labor drug (ritodrine, terbutaline, indomethacin or nifedipine). A second group of women in this study with advanced cervical dilation but not eligible for preterm labor drugs received either a 4 g dose of magnesium sulfate or a placebo (saline). The women who received the magnesium sulfate dose or doses had a higher rate of neonatal deaths (8 deaths out of 75 pregnancies) than the control group (1 death out of 75 pregnancies). The difference was statistically significant. (Mittendorf, Robert et. al, Lancet, Vol. 350, pp. 1517-1518.)

6. Will I be confined to my bed while on magnesium sulfate?

Yes, most women are. Your diet probably will be restricted to fluids because of the risk of vomiting. You may need to use a bedpan or a catheter.

7. Can magnesium sulfate be combined with other drugs?

Magnesium sulfate, when combined with nifedipine, can cause neuromuscular blockade (muscular paralysis). When combined with ritodrine or terbutaline, magnesium sulfate can greatly increase the risk of severe complications - including pulmonary edema and cardiovascular complications. A study of asthmatics found that combining magnesium sulfate and terbutaline increased terbutaline's cardiovascular side effects (Chest, 1994, Vol. 105, pp. 701-705).

8. Does magnesium sulfate work?

A 1998 review concluded that it is effective in preventing convulsions in women who have severe pre-eclampsia and in stopping convulsions in eclamptic women. The review consisted of 19 randomized, controlled trials, five retrospective studies and eight observational reports published in English between 1966 and February 1998. The review also concluded that more research is needed on whether magnesium sulfate is effective for women with mild pre-eclampsia and gestational high blood pressure (Obstetrics and Gynecology, Vol. 92, pp. 883-889).

9. Are there alternatives to magnesium sulfate in treating pre-eclampsia?

Magnesium sulfate is widely used in the United States to prevent convulsions from severe pre-eclampsia. In Europe and Australia, the most popular choices are: diazepam, phenytoin, chlormethiazole and barbituates.

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Treating Preterm Labor

1. Why is magnesium sulfate used in treating preterm labor?

When a woman experiences preterm labor (defined as contractions and cervical effacement or dilation before term), sometimes she is given magnesium sulfate, which is thought to relax the muscles of the uterus. It is not FDA approved for treating preterm labor and is used "off label" for this purpose. It was introduced in the United States as a preterm labor drug in 1969, on the basis of laboratory work. (Lancet, 1997, Vol. 350, p. 1517)

2. How is it administered?

It usually is given to patients through an IV in the hospital, and sometimes it is administered in shots. Some studies have used a pill form of magnesium sulfate to treat preterm labor, but the pill form appears to be experimental.

Dosages of magnesium sulfate vary quite a bit, depending on the doctor. In general, women experiencing preterm labor may receive higher doses for longer periods of time than women experiencing pre-eclampsia/eclampsia. Typically, a woman experiencing preterm labor will receive a loading (big) dose of about 4 to 6 grams through an IV. She then will receive a continuous dose through an IV of between 1 and 3 grams per hour. In some studies, the continuous dose was as high as 4 to 5 grams an hour.

3. How long do patients take magnesium sulfate?

In preterm labor, magnesium sulfate is given over 24 or 48 hours to try to halt labor. Sometimes, a woman will then be put on a different preterm labor drug like terbutaline or nifedipine and can be sent home. (For more information on terbutaline, click here.) Some women who are very far from term and whose condition is precarious may be kept on magnesium sulfate for longer periods of time - sometimes weeks and months.

4. What are the side effects of magnesium sulfate?

The line between a "therapeutic" and "toxic" dose of magnesium sulfate is very thin. In general, the larger the dose, the greater the chance of severe side effects. Doctors can test your blood to see how much magnesium is in it. This will indicate when a "safe" dose has been exceeded. They also can do reflex checks and monitor urine output, which also may indicate when a "safe" dose has been exceeded.

More common side effects

Complications (sometimes occurring with magnesium overdose)


Rare, Severe Complications

Adapted from: Hill, Washington Clark, "Risks and Complications of Tocolysis," Clinical Obstetrics and Gynecology, 1995, Vol. 38, No. 4, p. 732.

5. Does magnesium sulfate affect my baby or babies?

Yes, it crosses the placenta. After your baby or babies are born, they may have some of it in their blood.

Click here for a list of neonatal side effects.

In the past, magnesium sulfate has been thought to be safe for babies whose mothers take it. However, doctors are debating the significance of one 1997 study that challenges this view. More studies are needed before definite conclusions can be made.

This 1997 study was conducted at the University of Chicago and found an increased number of neonatal deaths in women who took it for preterm labor in comparison to those women who took another preterm labor drug (ritodrine, terbutaline, indemethacin or nifedipine). A second group of women in this study with advanced cervical dilation but not eligible for preterm labor drugs received either a 4 g dose of magnesium sulfate or a placebo (saline). The women who received the magnesium sulfate dose had a higher rate (8 deaths out of 75 pregnancies) of neonatal deaths than the control group (1 death out of 75 pregnancies). The differences were statistically significant. The preterm labor dosages of magnesium sulfate involved in this study were modest (4 gram loading dose and 2-3 grams per hour).(Mittendorf, Robert et. al, Lancet, Vol. 350, pp. 1517-1518.)

6. Will I be confined to my bed while on magnesium sulfate?

Yes, most women are. Your diet probably will be restricted to fluids because of the risk of vomiting. You may need to use a bedpan or a catheter.

7. Can magnesium sulfate be combined with other preterm labor drugs?

Yes, but there have been some complications. Magnesium sulfate, when combined with nifedipine, can cause neuromuscular blockade (muscular paralysis). When combined with ritodrine or terbutaline, it can greatly increase the risk of severe complications - including pulmonary edema and cardiovascular complications. A study of asthmatics found that combining magnesium sulfate and terbutaline increased terbutaline's cardiovascular side effects.

8. Does magnesium sulfate work?

Despite its widespread use, there are few studies on magnesium sulfate's use as a preterm labor drug. A 1997 review of published studies on magnesium sulfate found eight that met the reviewers' criteria. Of this eight, two compared magnesium sulfate to no treatment or a placebo. The other six compared magnesium sulfate to another preterm labor drug (either ritodrine or terbutaline). The review of studies (Obstetrical and Gynecology Survey, Vol. 52, pp. 652-658) concluded that more research -- with more test subjects -- is needed. It said the two placebo-controlled or no treatment controlled studies appear to show magnesium sulfate has no benefit. But these two studies may have been skewed by an insufficient number of study subjects. In reviewing the six studies comparing magnesium sulfate to terbutaline or ritodrine, the review concluded that magnesium sulfate appears to be as effective as ritodrine or terbutaline in delaying delivery for 48 hours. However, because of the small number of subjects involved in these studies, more research is necessary because there "may be clinically significant differences in efficacy between these agents." Click here for a table and study citations.

Two studies since 1997 compare magnesium sulfate to other preterm labor drugs. A 1999 study compared magnesium sulfate to the heart drug nitroglycerin in treating preterm labor (Obstetrics and Gynecology, Vol. 93, pp. 79-83). Its study involved 30 women and found magnesium sulfate to be more effective and with fewer serious side effects. A 1998 study involving 88 women compared magnesium sulfate to the prostaglandin synthetase inhibitor keterolac. It found keterolac stopped labor more quickly than magnesium sulfate.

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Questions to ask your doctor

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