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Drugs and Lactation Database (LactMed®) [Internet]. Bethesda (MD): National Institute of Child Health and Human Development; 2006-.

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Drugs and Lactation Database (LactMed®) [Internet].

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Magnesium Sulfate

Last Revision: January 15, 2024.

Estimated reading time: 3 minutes

CASRN: 7487-88-9

Drug Levels and Effects

Summary of Use during Lactation

Intravenous magnesium increases milk magnesium concentrations only slightly and oral absorption of magnesium by the infant is poor, so maternal magnesium therapy is not expected to affect the breastfed infant's serum magnesium. Although intravenous magnesium sulfate given prior to delivery might affect the infant's ability to breastfeed, intention to breastfeed may be a more important determinant of breastfeeding initiation.[1] Postpartum use of intravenous magnesium sulfate for longer than 6 hours appears to delay the onset of lactation.[2,3] One group of experts recommends reserving postpartum magnesium sulfate prophylaxis for those women with persistent neurologic symptoms within 7 days of birth.[4]

Drug Levels

Maternal Levels. Ten women with pre-eclampsia were given 4 grams of magnesium sulfate intravenously followed by 1 gram per hour until 24 hours after delivery. While the average serum magnesium was 35.5 mg/L in treated women compared to 18.2 mg/L in 5 untreated controls, colostrum magnesium levels at the time of discontinuation of the infusion was 64 mg/L in treated women and 48 mg/L in the controls. By 48 hours after discontinuation, colostrum magnesium levels were only slightly above control values and by 72 hours they were virtually identical to controls.[5]

Infant Levels. Relevant published information was not found as of the revision date.

Effects in Breastfed Infants

Relevant published information was not found as of the revision date.

Effects on Lactation and Breastmilk

One mother who received intravenous magnesium sulfate for 3 days for pregnancy-induced hypertension had lactogenesis II delayed until day 10 postpartum. No other specific cause was found for the delay, although a complete work-up was not done.[6] A subsequent controlled clinical trial found no evidence of delayed lactation in mothers who received intravenous magnesium sulfate therapy.[7] Some, but not all, studies have found a trend toward increased time to the first feeding or decreased sucking in infants of mothers treated with intravenous magnesium sulfate during labor because of placental transfer of magnesium to the fetus.[7,8] Another study found that among women with severe pre-eclampsia who received intravenous magnesium sulfate for up to one day postpartum and who intended to breastfeed, 85% of infants receiving routine well-baby care and 69% of those admitted to the NICU, breastfeeding was successfully initiated.[1]

A study randomized women with preeclampsia to receive intravenous magnesium sulfate for either 6 or 24 hours postpartum. There was no difference in the rate of eclampsia between the two groups. However, those who received the infusion for 24 hours had a delayed onset of lactation, 36.5 hours compared with 25.7 hours in the 6-hour group.[2]

A prospective, multicenter, randomized, controlled trial in 9 Latin American maternity hospitals compared patients with severe pre-eclampsia who had received at least 8 grams of magnesium sulfate prior to placebo. Patients were randomized to continue magnesium sulfate for 24 hours postpartum (n = 555) or stopping the infusion (n = 558). The time to lactation was significantly delayed in those who received magnesium sulfate postpartum (24.1 vs. 17.1 hours).[3]

A study randomized pregnant women with moderate to severe pre-eclampsia to receive magnesium sulfate intravenously infused at the same dose (not specified) for 8 or 24 hours. In patients who received the 8-hour infusion, the mean time to initiate breastfeeding was 14.6 hours compared to 24.3 hours in the patients who received the 24-hour infusion, which was a statistically significant difference.[9]

A retrospective chart review of mothers who delivered at the University of Chicago found that intravenous magnesium sulfate during delivery was associated with over 60% reduced odds of breastfeeding initiation compared to mothers who received no magnesium.[10]

Alternate Drugs to Consider

(Laxative) Docusate, Magnesium Hydroxide, Psyllium, Sodium Picosulfate, Sodium Phosphate


Cordero L, Valentine CJ, Samuels P, et al. Breastfeeding in women with severe preeclampsia. Breastfeed Med 2012;7:457-63. [PubMed: 22871169]
Vigil-DeGracia P, Ramírez R, Duran Y, Quintero A. Magnesium sulfate for 6 vs 24 hours post delivery in patients who received magnesium sulfate for less than 8 hours before birth: A randomized clinical trial. BMC Pregnancy Childbirth 2017;17:241. [PMC free article: PMC5525206] [PubMed: 28738788]
Vigil-DeGracia P, Ludmir J, Ng J, et al. Is there benefit to continue magnesium sulfate post-partum in women receiving magnesium sulfate prior to delivery? A randomized controlled study. BJOG 2018;125:1304-11. [PubMed: 29878650]
Cagino K, Prabhu M, Sibai B. Is magnesium sulfate theralpy warranted in all cases of late postpartum severe hypertension? A suggested approach to a clinical conundrum. Am J Obstet Gynecol 2023;229:641-6. [PubMed: 37467840]
Cruikshank DP, Varner MW, Pitkin RM. Breast milk magnesium and calcium concentrations following magnesium sulfate treatment. Am J Obstet Gynecol 1982;143:685-8. [PubMed: 7091241]
Haldeman W. Can magnesium sulfate therapy impact lactogenesis? J Hum Lact 1993;9:249-52. [PubMed: 8260059]
Riaz M, Porat R, Brodsky NL, Hurt H. The effects of maternal magnesium sulfate treatment on newborns: A prospective controlled study. J Perinatol 1998;18:449-54. [PubMed: 9848759]
Rasch DK, Huber PA, Richardson CJ, et al. Neurobehavioral effects of neonatal hypermagnesemia. J Pediatr 1982;100:272-6. [PubMed: 7199083]
Mushtaq S, Zafar M, Ahmed A, et al. Eight hours versus twenty four hours postpartum magnesium sulphate for prophylaxis in women with pre-eclampsia. Pakistan Journal of Medical and Health Sciences 2023;17:182-5. doi:10.53350/pjmhs2023174182 [CrossRef]
Kuriloff MA, Keenan-Devlin L, Betancourt GB, et al. Perinatal magnesium sulfate and human milk feeding initiation during postnatal hospitalization. Breastfeed Med 2023;18:A16. doi:10.1089/bfm.2023.29253.abstracts [CrossRef]

Substance Identification

Substance Name

Magnesium Sulfate

CAS Registry Number


Drug Class

Breast Feeding


Milk, Human




Gastrointestinal Agents

Magnesium Compounds


Tocolytic Agents

Disclaimer: Information presented in this database is not meant as a substitute for professional judgment. You should consult your healthcare provider for breastfeeding advice related to your particular situation. The U.S. government does not warrant or assume any liability or responsibility for the accuracy or completeness of the information on this Site.

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Attribution Statement: LactMed is a registered trademark of the U.S. Department of Health and Human Services.

Bookshelf ID: NBK501339PMID: 30000398


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