MSM (methylsulfonylmethane) has not been proven safe or unsafe during pregnancy in humans, because no clinical trials have been conducted in pregnant women. The limited evidence available comes from animal studies, which showed no harm to developing offspring even at high doses. That puts MSM in a gray zone: not flagged as dangerous, but without the human data needed to give it a clear green light.
What Animal Studies Show
The most relevant safety data comes from a developmental toxicity study in rats. Pregnant rats received MSM by mouth during days 6 through 20 of gestation at doses up to 1,000 mg per kilogram of body weight per day. Researchers found no evidence of harm to the mothers or their offspring. Litter size, litter viability, and fetal body weight were all unaffected. No malformations or structural abnormalities appeared in any of the fetuses, and there was no increase in fetal mortality. The study set the no-observed-adverse-effect level (NOAEL) at 1,000 mg/kg/day, meaning that was the highest dose tested and it still caused no problems.
A separate toxicology review identified a NOAEL of over 1.5 g/kg in animals, which translates to a human equivalent dose of roughly 14.5 grams per day for a 130-pound person. That’s well above what most people take as a supplement. Typical MSM doses in studies range from 1.5 to 6 grams per day, so the margin between a normal supplement dose and the level that caused no harm in animals is wide.
Animal data is reassuring but has limits. Rats metabolize substances differently than humans, and the studies don’t capture the full complexity of human pregnancy. This is why the absence of human trials matters.
MSM’s Regulatory Status
The FDA accepted MSM as “Generally Recognized As Safe” (GRAS) for use in certain foods, including meal replacement products, smoothie-type drinks, and energy bars, at specified levels. This GRAS determination covers general food use, not pregnancy specifically. The FDA’s acceptance was based on toxicology data and the company’s own safety conclusions, and the agency noted it had “no questions” about those conclusions. However, the FDA also clarified that its letter was not a formal affirmation of GRAS status, which is an important distinction: it means the FDA didn’t object rather than actively certifying safety.
No major health authority has issued specific guidance saying MSM is safe to take as a supplement during pregnancy, nor has any issued a formal warning against it.
What About Breastfeeding?
MSM is naturally present in human breast milk in small amounts. It’s a normal metabolite found in the bloodstream and in milk. A study of milk samples from four healthy nursing women found that MSM levels were actually lower in human breast milk than in cow’s milk-based infant formulas. A larger Finnish study of 429 mothers measured milk levels of MSM between 0.9 and 1.2 micrograms per liter, which is a very small quantity.
No studies have directly examined what happens when a breastfeeding mother takes MSM supplements, but the National Institutes of Health’s LactMed database notes that its low toxicity profile suggests it is “unlikely to harm the breastfed infant.” Still, no data exists on infant blood levels after maternal supplementation.
Why the Uncertainty Exists
MSM sits in the same category as many dietary supplements: widely used, low apparent toxicity, but studied almost exclusively in non-pregnant adults. The clinical trials that do exist focused on joint pain in people with osteoarthritis or on exercise recovery in athletes. These studies used doses of 1.5 to 6 grams daily for up to 12 weeks and generally reported mild side effects like digestive discomfort, but pregnant women were not enrolled in any of them.
This gap isn’t unique to MSM. Ethical restrictions make it difficult to run supplement trials in pregnant women, so the safety profile for most supplements during pregnancy relies on animal data, case reports, and observational evidence rather than controlled human studies.
Alternatives With Stronger Pregnancy Data
If you’re considering MSM for joint pain or leg cramps during pregnancy, a few alternatives have more established safety profiles in pregnant populations.
- Magnesium is considered very safe during pregnancy and may help with leg cramps. The tolerable upper limit for supplementation is 350 mg per day in the United States. Side effects above that level are typically limited to mild diarrhea, and toxic levels only occur at oral doses above 2,500 mg per day.
- Vitamin E has shown benefit for pregnancy-related leg cramps in smaller studies at doses around 100 mg per day. The tolerable upper limit is 1,000 mg per day in the U.S., with the main concern being a possible effect on blood clotting at very high doses.
- Vitamin K2 has been used safely for pregnancy-associated bone loss at higher therapeutic doses, and no toxicity level has been established for it.
These options aren’t perfect substitutes for MSM, especially if you’re taking it for a specific purpose like joint support, but they come with more direct evidence of safety during pregnancy. If MSM is part of your current routine and you’ve recently become pregnant or are planning to, the decision comes down to weighing a reassuring but incomplete safety picture against the option of pausing until after delivery.