Magnesium oxide 400 mg is one of the most commonly sold magnesium supplements, used primarily for three purposes: relieving constipation, preventing migraines, and treating heartburn. A 400 mg tablet contains about 241 mg of elemental magnesium, which is roughly 60% of the daily value most adults need. Despite being one of the less well-absorbed forms of magnesium, its high elemental magnesium content and low cost make it a popular choice.
Constipation Relief
The most straightforward use for magnesium oxide 400 mg is as an osmotic laxative. When you swallow the tablet, it reacts with stomach acid and eventually reaches the small intestine, where it draws water from the intestinal walls into the digestive tract. This extra water softens stool and expands the intestinal contents, which stimulates a bowel movement. Because magnesium oxide is poorly absorbed into the bloodstream, most of it stays in the gut, which is exactly what makes it effective for this purpose.
A randomized, placebo-controlled trial published in the Journal of Neurogastroenterology and Motility confirmed that magnesium oxide significantly increased the frequency of bowel movements in patients with chronic constipation compared to placebo. For occasional constipation, the 400 mg dose is a common starting point, though some people need more or less depending on their response.
Migraine Prevention
Magnesium oxide at 400 mg daily is widely used as a preventive treatment for migraines, not as a painkiller once one starts. People with migraines tend to have lower magnesium levels, and supplementing appears to reduce how often attacks occur. In a study published in the American Journal of Obstetrics and Gynecology looking at migraine prevention during pregnancy, patients who took magnesium oxide saw their median monthly migraine frequency drop from 16 episodes to just 1. Fifty-nine percent of patients in that study were treated successfully with magnesium oxide alone, without needing additional medications.
The effect isn’t instant. Most people need to take magnesium oxide consistently for several weeks before noticing fewer migraines. This is a preventive strategy, meaning it works by changing your baseline over time rather than stopping an individual headache.
Heartburn and Acid Indigestion
Magnesium oxide also works as an antacid. It directly neutralizes stomach acid, which can provide short-term relief from heartburn, sour stomach, or acid indigestion. This is a different mechanism from its laxative effect: here, it’s the chemical reaction with gastric acid that does the work. If you’re using magnesium oxide specifically as an antacid, it shouldn’t be taken for longer than two weeks at a time without medical guidance, since persistent heartburn may signal something that needs a different approach.
Why Absorption Matters
Magnesium oxide has a reputation for being one of the least bioavailable forms of magnesium, meaning your body absorbs a smaller percentage of it compared to other forms. Magnesium citrate, glycinate, malate, and chloride are all more easily absorbed in the digestive tract. This is why magnesium oxide isn’t typically the first choice for correcting an actual magnesium deficiency.
That said, magnesium oxide packs more elemental magnesium per milligram than most other forms. A 400 mg tablet delivers 241 mg of elemental magnesium, whereas you’d need a much larger dose of magnesium glycinate or citrate to get the same amount. For gut-related uses like constipation, lower absorption is actually an advantage, since the magnesium that stays in your intestines is what produces the laxative effect. For migraine prevention, the clinical evidence supports its effectiveness at this dose despite the absorption trade-off.
How It Compares to Your Daily Needs
Most adult men need 400 to 420 mg of total magnesium per day, and most adult women need 310 to 320 mg. These numbers include magnesium from food, not just supplements. A single 400 mg magnesium oxide tablet provides about 241 mg of elemental magnesium, which covers a significant portion of your daily requirement on its own, before counting what you get from nuts, leafy greens, whole grains, and other dietary sources.
The tolerable upper intake level set by the National Institutes of Health for supplemental magnesium is 350 mg per day for adults. This limit applies only to magnesium from supplements and medications, not from food. Since a 400 mg magnesium oxide tablet contains 241 mg of elemental magnesium, it falls within this threshold. Going significantly above it increases the risk of digestive side effects, particularly diarrhea and cramping.
Side Effects and Digestive Tolerance
The most common side effects of magnesium oxide are loose stools, diarrhea, and stomach cramps. These are dose-dependent: the more you take, the more likely they are. For many people, 400 mg is well tolerated, but if you experience persistent diarrhea, reducing the dose or splitting it across two smaller doses during the day can help. Taking it with food also tends to reduce stomach upset.
Who Should Be Cautious
People with kidney problems need to be especially careful with magnesium oxide. Healthy kidneys efficiently filter excess magnesium from the blood, but impaired kidneys cannot. For people with moderate kidney impairment (a filtration rate between 30 and 59), doses typically need to be reduced by 25 to 50 percent. For severe kidney impairment (filtration rate below 30), magnesium oxide should generally be avoided or used only under close medical supervision, since magnesium can build up to dangerous levels in the blood.
Magnesium oxide also interferes with the absorption of several common medications. Bisphosphonates, which are used for osteoporosis, lose significant effectiveness when taken alongside magnesium. In one example, a bisphosphonate’s absorption dropped by 60% when taken within an hour of a magnesium-containing antacid. Certain antibiotics, thyroid medications, and other drugs have similar interactions. If you take any prescription medication, spacing it at least two hours apart from your magnesium oxide dose is a practical rule of thumb to minimize interference.