Magnesium Hydroxide: what it does, how to use it, and when to be careful

Magnesium hydroxide shows up in many first-aid cabinets as "milk of magnesia." People use it for heartburn and for constipation because it neutralizes stomach acid and draws water into the gut. That sounds simple, but knowing how it works and how to use it safely makes a big difference—especially if you use it more than once in a while.

How it works and common uses

As an antacid, magnesium hydroxide neutralizes excess stomach acid on contact. You usually feel relief within minutes, and the effect can last a few hours. As a laxative, it acts as an osmotic agent: it pulls water into the intestines, softening stool and speeding up bowel movements. Laxative effects often start in 30 minutes to 6 hours, depending on dose and your digestive speed.

Common uses:

• Heartburn, sour stomach, or indigestion (short-term relief)
• Occasional constipation when you need quick relief
• Sometimes used in combination products for other stomach issues

Safety, dosing and buying tips

Dosage varies by product and age—always read the label. Typical adult antacid doses are small, taken after meals or at bedtime for quick relief. For constipation, over‑the‑counter milk of magnesia labels often recommend 30–60 mL (1–2 tablespoons) once daily for adults, but follow the bottle or your pharmacist’s advice. For kids, use the lower doses listed on pediatric labels or ask a pharmacist.

Watch for these common side effects: diarrhea, stomach cramps, and loose stools. If you get severe diarrhea, stop using it. People with kidney disease must avoid regular use—magnesium can build up and cause serious problems (weakness, breathing trouble, very low blood pressure). If you take prescription drugs, separate doses: magnesium can reduce absorption of antibiotics like tetracyclines and fluoroquinolones, and can interfere with some thyroid medicines. Give a 2–4 hour gap when possible.

Long-term daily use isn’t a good idea unless a doctor recommends it. Frequent use can disturb your electrolytes (magnesium, sodium) and make bowel habits worse. If constipation lasts more than a week, or you see blood in stool, get medical advice.

Buying tips: magnesium hydroxide is OTC in many countries, so you can buy it without a prescription. When buying online, pick a reputable pharmacy, check product labels, and read recent customer reviews. Make sure the site shows contact info and pharmacy licensing. If you’re unsure about mixing it with other meds you take, ask your pharmacist before ordering.

Storage is simple: keep the bottle closed, store at room temperature, and keep it out of reach of children. For quick relief of heartburn or occasional constipation, magnesium hydroxide works well—just use it responsibly and check with a pro if you have kidney issues or take multiple medicines.