Magnesium citrate is a saline laxative that draws water into your intestines, softening stool and triggering bowel movements. It’s one of the most commonly used over-the-counter solutions for occasional constipation and is also used to prepare the bowel before medical procedures like a colonoscopy. Beyond its laxative effect, magnesium citrate serves as a well-absorbed form of magnesium supplementation and may help reduce the risk of kidney stones.
How It Works in Your Gut
Magnesium citrate belongs to a class of medications called osmotic (or saline) laxatives. When you drink it, the magnesium pulls water into your intestines through osmosis. That extra fluid does two things: it softens stool so it’s easier to pass, and it increases the volume inside your intestines, which stimulates the muscles lining your bowel to contract and move things along.
The effect is fast. Most people experience bowel movements within 30 minutes to one hour of taking it, though it can sometimes take a few hours depending on what’s in your stomach and how much you drank. Because the timeline is so short, it’s best taken when you’ll be near a bathroom for a few hours afterward.
Common Uses
Relieving Occasional Constipation
The standard adult dose for constipation is 6.5 to 10 fluid ounces of the oral solution, with a maximum of 10 fluid ounces in 24 hours. Each fluid ounce contains about 290 mg of magnesium. Children ages 6 to 11 can take 3 to 7 fluid ounces, and children ages 2 to 5 are limited to 2 to 3 fluid ounces. This is meant for short-term, occasional use, not as an everyday laxative.
Colonoscopy Bowel Preparation
Gastroenterologists frequently include magnesium citrate in colonoscopy prep routines. A typical protocol, like the one used at Cleveland Clinic, calls for drinking a full 10-fluid-ounce bottle two days before the procedure, usually in the late afternoon. It’s combined with other prep agents and a clear liquid diet to fully empty the bowel. If you’ve been prescribed a colonoscopy prep that includes magnesium citrate, follow the specific timing your doctor’s office provides, since protocols vary.
Magnesium Supplementation
Many people take magnesium citrate not for its laxative effect but simply to increase their magnesium levels. Organic forms of magnesium (those bonded to a carbon-containing molecule, like citrate) are better absorbed by the body than inorganic forms like magnesium oxide. This makes citrate a popular choice for addressing magnesium deficiency, supporting muscle function, and promoting sleep. One important detail: the percentage of magnesium your body actually absorbs decreases as the dose goes up, so splitting a daily dose into smaller amounts taken throughout the day can improve absorption.
Kidney Stone Prevention
Magnesium citrate has a notable benefit that goes beyond digestion. The citrate component binds to calcium in urine, forming a soluble complex that prevents calcium from clumping with oxalate, the main ingredient in the most common type of kidney stone. Citrate also makes urine less acidic, which further discourages stone formation. Meanwhile, the magnesium itself slows the rate at which calcium oxalate crystals grow.
The evidence for this is striking. In a clinical trial published in The Journal of Urology, 63.6% of participants taking a placebo developed new kidney stones over the study period, compared to just 12.9% of those taking potassium-magnesium citrate. That’s an 85% reduction in recurrence risk over up to three years of treatment. For people with a history of calcium oxalate stones, this combination is one of the more effective preventive strategies available.
Side Effects and Risks
At standard laxative doses, the most common side effects are cramping, bloating, and diarrhea. These are essentially the intended mechanism working a bit too aggressively. Drinking it cold and following it with a full glass of water can reduce nausea.
The bigger concern is what happens with overuse or high doses. Magnesium citrate can cause dehydration, since it pulls a significant amount of water into the intestines. It can also shift your electrolyte balance, affecting blood levels of sodium, calcium, and potassium. In extreme cases, too much magnesium in the bloodstream (hypermagnesemia) can cause low blood pressure, muscle weakness, and irregular heartbeat. These risks are low with occasional, recommended-dose use but become real if someone uses it frequently or takes more than directed.
People with kidney disease face a specific risk. Healthy kidneys filter excess magnesium out of the blood efficiently, but kidneys that aren’t functioning well can’t keep up. Magnesium and other mineral ingredients in over-the-counter laxatives and antacids can build up to dangerous levels in people with chronic kidney disease. The National Kidney Foundation advises checking with a healthcare provider before using magnesium-containing products if you have any degree of kidney impairment.
Tips for Taking It
Magnesium citrate oral solution has a strong, salty-sour taste that most people find unpleasant. Refrigerating it before drinking makes it more tolerable. Many people chase it with water or mix it with a clear flavored beverage. Drinking a full glass of water afterward is also important because of the fluid your intestines will pull from your body.
For constipation relief, take it on a relatively empty stomach for the fastest results. Avoid using it for more than a week without medical guidance, since chronic use can make your bowels dependent on laxatives to function normally. If you’re taking it as a daily magnesium supplement in pill or capsule form (rather than the liquid laxative), the doses are much smaller and the laxative effect is usually mild or absent, though loose stools can still occur if you take too much at once.