Magnesium oxide and magnesium citrate are two of the most common magnesium supplements on store shelves, but they differ significantly in how well your body absorbs them, how they affect your gut, and what they’re best used for. The biggest distinction: magnesium citrate is absorbed more completely into your bloodstream, while magnesium oxide delivers more elemental magnesium per pill but much of it passes through unabsorbed.
How Absorption Differs
Magnesium oxide contains roughly 60% elemental magnesium by weight, the highest concentration of any common supplement form. A standard 400 mg tablet packs a lot of magnesium into a single pill. The problem is that your body only absorbs a fraction of it. Studies have found that magnesium in citrate form is absorbed more completely and is more bioavailable than magnesium oxide. “Bioavailable” simply means more of what you swallow actually reaches your cells.
This creates a bit of a paradox. Magnesium oxide looks better on the label because each pill contains more milligrams, but magnesium citrate delivers more usable magnesium per dose. If your goal is to raise your magnesium levels or correct a deficiency, citrate is generally the more efficient choice. Some kidney health specialists specifically recommend magnesium citrate, glycinate, or malate over oxide for supplementation because of this absorption advantage.
Digestive Side Effects
Both forms can cause diarrhea, but they’re not equally likely to do so. The NIH lists magnesium oxide among the forms most commonly reported to cause diarrhea. The reason is straightforward: the magnesium your body doesn’t absorb stays in your intestines, where it pulls water in through osmosis. Since oxide has lower absorption, more of it remains in the gut, which means more water gets drawn in and stools loosen.
Magnesium citrate also has a well-documented laxative effect, especially at higher doses. It draws water into the small intestine, causing fluid accumulation and eventually a bowel movement. In liquid form, magnesium citrate is actually used as a bowel prep before medical procedures. But at typical supplement doses, citrate tends to cause less GI upset than oxide precisely because more of it gets absorbed before reaching the lower intestine. If you’ve tried magnesium oxide and found it hard on your stomach, switching to citrate often helps.
When Laxative Effects Are the Goal
If you’re dealing with constipation rather than trying to boost your magnesium levels, the laxative properties of these supplements become a feature rather than a bug. Magnesium citrate in liquid form is one of the most widely used over-the-counter options for occasional constipation. It typically produces a bowel movement within 30 minutes to 6 hours after you drink it, making it relatively fast-acting.
Magnesium oxide also has recognized laxative uses, though it’s less commonly chosen for this purpose. Both work through the same osmotic mechanism: unabsorbed magnesium salts pull water into the intestines and stimulate the gut to move things along. Clinicians generally caution against using either form if loose stools are already a problem, since both can make diarrhea worse.
Migraine Prevention
One area where magnesium oxide has a specific, well-established role is migraine prevention. The American Headache Society recommends 400 to 500 milligrams of magnesium oxide daily as a preventive measure for people with frequent migraines. This is one of the few situations where oxide’s high elemental magnesium content per pill is a practical advantage, since it lets you reach that 400-plus milligram target with fewer tablets.
Magnesium citrate can also support migraine prevention, but oxide is the form most commonly studied and recommended for this use. If you tolerate it well digestively, oxide at the recommended dose is a reasonable and inexpensive option for reducing migraine frequency.
Cost and Availability
Both forms are widely available and inexpensive. Magnesium oxide tablets (120-count bottles at 400 mg strength) typically run between $6 and $8 without insurance. Magnesium citrate in liquid form costs around $5.50 for a 296 mL bottle. Capsule and tablet forms of citrate vary more in price but remain affordable. Neither supplement will strain your budget, so cost alone isn’t a strong reason to pick one over the other.
Which Form to Choose
Your choice comes down to what you’re trying to accomplish. For raising magnesium levels or addressing a deficiency, magnesium citrate is the stronger option because your body absorbs more of it with fewer digestive complaints. For migraine prevention specifically, magnesium oxide at 400 to 500 mg daily has the most established track record. For occasional constipation relief, liquid magnesium citrate works quickly and predictably.
If you’ve been taking magnesium oxide for general health and experiencing cramping, nausea, or loose stools, poor absorption is the likely culprit. Switching to citrate often solves the problem while actually delivering more magnesium to your tissues. People with kidney disease should be cautious with any magnesium supplement, since impaired kidneys have a harder time clearing excess magnesium from the blood, but citrate is generally considered the safer supplemental form due to its better absorption profile and lower risk of GI complications.