Magnesium Hydroxide (Milk of Magnesia): Uses, Dose, and Safety

Magnesium hydroxide is a simple, widely used medicine you probably know as Milk of Magnesia. People take it for heartburn, sour stomach, and to relieve occasional constipation. It works quickly, is available over the counter, and is cheap. Here’s what you need to know to use it safely and effectively.

How it works and common uses

Magnesium hydroxide does two jobs. In the stomach it neutralizes excess acid and eases indigestion. In the gut it pulls water into the bowel, which softens stool and helps produce a bowel movement. As an antacid you may feel relief within minutes. As a laxative expect effects from about 30 minutes up to 6 hours depending on dose and individual response.

Typical uses include occasional heartburn, acid upset, and short-term relief from constipation. It’s not meant for daily long-term constipation treatment unless your doctor advises it. You’ll find it as a liquid suspension labeled Milk of Magnesia and sometimes in chewable tablets or capsules that contain other magnesium salts.

Safety, dosing and practical tips

Follow the product label or your doctor’s instructions. A common adult antacid dose is 5–15 mL (1–3 teaspoons) as needed. For constipation, many over-the-counter directions recommend 30–60 mL (1–2 tablespoons) as a single dose. Doses for children are lower and based on age—use a pediatric product or ask your pharmacist.

Common side effects are mild: diarrhea and abdominal cramping. Diarrhea is actually how the laxative effect works, so reduce dose if it’s too loose. People with kidney disease should avoid magnesium-containing laxatives unless a doctor says it’s safe. Kidneys clear excess magnesium; when they don’t work well, magnesium can build up and cause serious problems like muscle weakness, very low blood pressure, slow breathing, or drowsiness.

Magnesium can interfere with some medicines. It binds certain antibiotics (tetracyclines, fluoroquinolones) and reduces absorption. It can also affect bisphosphonates and thyroid meds. To be safe, take other oral medicines at least 2 hours before or 4 hours after magnesium hydroxide unless your prescriber says otherwise.

Pregnant or breastfeeding? Short-term use for occasional constipation or heartburn is generally considered okay but check with your clinician. For children, follow pediatric dosing and avoid giving adult doses to young kids.

Stop using and call a doctor if you get severe or bloody diarrhea, no bowel movement after 48–72 hours despite treatment, signs of magnesium overdose, or if heartburn/indigestion persists despite treatment. Store the product at room temperature, keep the cap tightly closed, and keep it away from children.

Want a quick tip? Try a lower dose first to see how your body reacts. If it works, stick with that. If not, talk to your pharmacist or doctor about safer long-term options for chronic constipation or frequent heartburn.