Magnesium citrate, magnesium hydroxide (Milk of Magnesia), and magnesium oxide are the three forms most commonly used for constipation. All three work the same way: magnesium ions are poorly absorbed in the gut, so they pull water into the intestines through osmosis, softening stool and triggering movement. The difference comes down to how well each form works, how fast it acts, and how easy it is to find.
Magnesium Citrate
Magnesium citrate is the most popular choice for constipation relief and the one most often recommended by pharmacists. It comes as a liquid (usually sold in 10-ounce bottles at any drugstore) or as tablets. The liquid version works fast, typically producing a bowel movement within 30 minutes to 6 hours. That speed makes it useful for both occasional constipation and bowel prep before medical procedures.
Magnesium citrate also has better bioavailability than magnesium oxide, meaning your body absorbs more of it. That might sound counterintuitive for a laxative, since unabsorbed magnesium is what draws water into the bowel. But the higher absorption rate means citrate delivers its effect more predictably at lower doses, with less risk of overwhelming your system. If you’re looking for one form to try first, citrate is a solid starting point.
Magnesium Hydroxide (Milk of Magnesia)
Milk of Magnesia has been a medicine cabinet staple for decades. It’s a liquid suspension of magnesium hydroxide, and like citrate, it typically works within 30 minutes to 6 hours. The standard adult dose is 2 to 4 tablespoons taken with a full glass of water, preferably at bedtime. Children aged 6 to 11 use roughly half that amount.
One advantage of magnesium hydroxide is its dual purpose: at lower doses it works as an antacid, while higher doses produce a laxative effect. The taste can be chalky, though flavored versions help. It’s widely available over the counter and inexpensive, making it a practical option for occasional use.
Magnesium Oxide
Magnesium oxide is the form you’ll find most often in supplement capsules and tablets. It has the lowest bioavailability of the common forms, with only about 4% of the magnesium actually absorbed into the bloodstream. Compare that to magnesium citrate, chloride, or lactate, which are absorbed at rates between 9 and 11%. That poor absorption means more magnesium stays in your intestines, which sounds like it should make oxide the strongest laxative. In practice, it still works well. A clinical trial found that magnesium oxide improved constipation symptoms in about 68% of participants, nearly identical to the 69% response rate seen with senna, a stimulant laxative. Time to first bowel movement averaged around 18 hours.
The American Gastroenterological Association and American College of Gastroenterology include magnesium oxide in their constipation management guidelines, though it carries a note of caution for people with kidney problems. If you already take a magnesium oxide supplement for general health, its laxative effect may be something you’ve noticed. That’s the trade-off: oxide is easy to find in pill form, but its gut effects can be unpredictable if you’re taking it for reasons other than constipation.
Forms That Won’t Help With Constipation
Not every magnesium supplement has a laxative effect. Magnesium glycinate is specifically designed to minimize gastrointestinal side effects. It’s better tolerated by people who already deal with loose stools, making it a poor choice if constipation relief is your goal. Magnesium glycinate is more commonly used for general magnesium supplementation, sleep support, or mood, though evidence for those benefits in humans is still limited.
Magnesium L-threonate, taurate, and malate are similarly well-absorbed forms that largely bypass the gut. If you see these on a supplement label, they’re not going to do much for your bowel habits. For constipation, you specifically want a poorly absorbed form that stays in the intestines long enough to draw in water.
How to Use Magnesium Safely
For occasional constipation, magnesium citrate or Milk of Magnesia taken with a full glass of water is the standard approach. Most people get relief within a few hours. Take it at bedtime if you’d prefer results by morning. Stay near a bathroom, especially with citrate liquid, which can work quickly.
Common side effects include cramping, nausea, and (unsurprisingly) diarrhea if you take too much. These are usually mild and resolve on their own. The more serious risk is a condition called hypermagnesemia, where magnesium builds up in the blood. In mild cases this causes headache, nausea, and drowsiness. Severe cases can lead to dangerously slow heart rhythms or respiratory failure. This is rare in people with healthy kidneys, because the kidneys efficiently clear excess magnesium.
People with chronic kidney disease face the greatest risk. Impaired kidneys can’t excrete magnesium fast enough, and even moderate doses of magnesium laxatives can push blood levels into a dangerous range. Older adults and anyone using magnesium laxatives regularly for weeks or months should also be cautious. If you have reduced kidney function, talk to your doctor before using any magnesium-based laxative.
Timing Around Other Medications
Magnesium can interfere with the absorption of several common medications. Antibiotics like tetracyclines and fluoroquinolones bind to magnesium in the gut, reducing their effectiveness. Bone-density medications (bisphosphonates like alendronate) are particularly sensitive: magnesium should be taken at least 30 minutes after the bisphosphonate dose to avoid blocking absorption. As a general rule, separate magnesium from other medications by at least two hours unless you’ve confirmed there’s no interaction.
Quick Comparison
- Magnesium citrate: Fastest acting (30 min to 6 hours), available as liquid or tablets, higher bioavailability, best first choice for most people.
- Magnesium hydroxide: Same speed as citrate, liquid form (Milk of Magnesia), doubles as an antacid at lower doses, widely available and inexpensive.
- Magnesium oxide: Slowest of the three (results around 18 hours), lowest absorption, commonly sold in capsule form, effective but less predictable timing.
- Magnesium glycinate, threonate, taurate: Well absorbed, minimal laxative effect, not useful for constipation.