Using Magnesium Hydroxide (Milk of Magnesia): Doses, Uses & Safety

Magnesium hydroxide—often sold as "milk of magnesia"—works two ways: as an antacid to neutralize stomach acid and as an osmotic laxative to relieve constipation. It's cheap, fast, and widely available over the counter. Below I’ll explain how to use it, common doses, safety warnings, and what to watch for so you can use it without surprises.

How to use it

For heartburn or indigestion, people usually take a small dose when symptoms start. Follow the label, but common short-term antacid doses are a few milliliters as needed after meals or at bedtime. For constipation, magnesium hydroxide draws water into the bowel and softens stool; it’s usually taken once daily or as directed on the bottle. Many adults take it at bedtime so it works by morning.

Always shake the bottle well. Measure liquid doses with a proper dosing cup or syringe—kitchen spoons aren’t accurate. If you take other meds, separate magnesium hydroxide from many prescription drugs by 2 hours: take the other drug either 2 hours before or 2 hours after the antacid. That helps prevent reduced absorption.

Safety tips & interactions

Don’t mix magnesium hydroxide with certain medications. It can bind and reduce absorption of antibiotics like tetracyclines or fluoroquinolones, bisphosphonates (for bone health), levothyroxine, iron supplements, and some antivirals. If you take one of those, check drug instructions or ask a pharmacist about timing.

Kidney function matters. If you have chronic kidney disease, your body may not clear extra magnesium well. That raises the risk of high magnesium (hypermagnesemia), which causes weakness, low blood pressure, slowed breathing, or confusion. People with kidney problems should avoid regular use unless a doctor recommends it.

Side effects are usually mild: loose stools, cramping, or gas. If you get severe diarrhea, stop using it and contact a provider. Signs of magnesium overdose include very low blood pressure, severe drowsiness, trouble breathing, or irregular heartbeat. Seek emergency care if those occur.

Children need special dosing—don’t give adult doses to kids. Pregnant and breastfeeding people can use magnesium hydroxide occasionally for constipation, but ask a clinician if you’re unsure or on other medications. Long-term daily use of any laxative isn’t ideal; talk to your doctor if you need it for more than a week.

Store the bottle at room temperature, keep it sealed, and check the expiration date. If symptoms don’t improve in a few days, you have blood in your stool, severe stomach pain, or unexplained weight loss, see a healthcare provider. Magnesium hydroxide helps a lot of people, but using it the right way keeps it safe and effective.