Citrate of magnesia, more commonly called magnesium citrate, is a saline laxative sold over the counter as a liquid solution. It works by drawing water into the intestines through osmosis, which softens stool and triggers a bowel movement, typically within 30 minutes to 6 hours. You’ll find it in the laxative aisle of most pharmacies in 10-ounce bottles, usually flavored with lemon or cherry to mask its salty, tart taste.
How It Works in Your Body
Magnesium citrate is an “osmotic” laxative. When you drink it, the magnesium ions aren’t easily absorbed through your intestinal walls. Instead, they stay in the intestinal space and pull water in after them, the same way salt draws moisture out of food. This flood of extra fluid increases the volume and softness of everything in your intestines, which stretches the intestinal walls and stimulates the muscles to push contents along.
The result is a watery bowel movement, sometimes several. Most people experience their first bowel movement roughly 1 to 3 hours after drinking the solution, though the full effect can take up to 6 hours. Once the process starts, bowel activity may continue for several hours before settling down.
Common Uses
The most familiar use is relieving occasional constipation. It’s not meant for daily or long-term use. A short course clears out a backup, but relying on it regularly can disrupt your body’s electrolyte balance and make your bowels dependent on stimulation to move.
Doctors also prescribe magnesium citrate as part of bowel preparation before a colonoscopy. In that context, the goal is to completely empty the colon so the doctor has a clear view. A typical colonoscopy prep involves two 10-ounce bottles: one the evening before the procedure and the second about six hours before your appointment time, each followed by 32 ounces of clear fluids. The split dosing is more effective and more tolerable than drinking everything at once.
Dosage for Constipation
For occasional constipation in adults and children 12 and older, the standard dose is 6.5 to 10 fluid ounces, with a maximum of 10 ounces in 24 hours. Children ages 6 to 11 can take 3 to 7 fluid ounces (7 ounces max per day), and children ages 2 to 5 can take 2 to 3 fluid ounces (3 ounces max per day). Children under 2 should not take it without a doctor’s guidance.
You should drink a full 8-ounce glass of water with each dose. Staying well hydrated matters because the solution is pulling water into your intestines, and without enough fluid intake, you risk dehydration. Many people find it easier to drink when it’s chilled.
Side Effects
The most common side effects are cramping, bloating, nausea, and loose or watery stools. These are essentially the intended effect dialed up a notch and usually resolve within a few hours. Diarrhea that persists can lead to dehydration and loss of important electrolytes like sodium and potassium, so replacing fluids is essential.
More serious reactions are rare but possible when too much magnesium builds up in the bloodstream. Signs of magnesium toxicity include muscle weakness, drowsiness, confusion, irregular heartbeat, and dangerously low blood pressure. In extreme cases, very high magnesium levels can slow breathing or cause cardiac arrest. These severe outcomes are almost exclusively linked to overdose or to use in people whose kidneys can’t clear the extra magnesium.
Who Should Avoid It
People with kidney disease face the highest risk. Healthy kidneys filter out excess magnesium efficiently, but impaired kidneys retain it. Even a single standard dose can push magnesium levels high enough to cause serious symptoms in someone with advanced kidney disease. One clinical case described in the American Journal of Kidney Diseases involved a patient with end-stage kidney disease who used an over-the-counter magnesium laxative and required emergency dialysis to bring levels back to normal.
You should also avoid magnesium citrate if you have signs of a bowel obstruction (severe abdominal pain, vomiting, no passage of gas), since forcing fluid into a blocked intestine can be dangerous. People on a low-sodium or restricted-magnesium diet, and anyone already experiencing diarrhea or dehydration, should skip it as well.
Drug Interactions to Know About
Magnesium citrate can interfere with how your body absorbs certain medications. It reduces the effectiveness of several common antibiotics, including those in the fluoroquinolone and tetracycline families, by binding to the drug in your gut before it can be absorbed. It also affects absorption of medications used for bone density and the heart medication digoxin. If you take any prescription medications, spacing them at least two hours away from a magnesium citrate dose helps minimize this effect.
How It Compares to Other Magnesium Forms
Magnesium comes in many supplement forms, and the type of molecule attached to the magnesium changes what it does best. Magnesium citrate is an organic form with relatively high solubility, meaning it dissolves and absorbs better than inorganic forms like magnesium oxide. That higher solubility is part of why it’s so effective at pulling water into the gut.
Other organic forms, like magnesium glycinate, are designed to be absorbed into the bloodstream more completely with less laxative effect. People who want to correct a magnesium deficiency or support sleep and muscle relaxation typically choose glycinate or similar forms for that reason. Magnesium citrate sits in a middle ground: it does contribute some absorbable magnesium to your body, but its primary action is in the gut. If your goal is a magnesium supplement without the bathroom trips, citrate is probably not your best choice. If you need reliable, fast-acting constipation relief, it’s one of the most accessible options available.
One additional benefit noted in research: magnesium citrate can reduce the formation of calcium oxalate crystals and lower oxalate absorption in the intestines, which is why some doctors recommend it for people prone to certain types of kidney stones.