Aluminum Hydroxide/Magnesium Hydroxide Drug Interactions: What to Avoid and What to Tell Your Doctor

One of the most underappreciated facts about OTC antacids like aluminum hydroxide/magnesium hydroxide (Mylanta) is how many prescription and OTC medications they interact with. With 445 documented drug interactions — including 19 major ones — this "harmless" antacid can significantly affect the effectiveness of other medications you're taking. Here's what you need to know.

How Does Aluminum Hydroxide/Magnesium Hydroxide Interact with Other Drugs?

The vast majority of interactions occur through one primary mechanism: this antacid alters the environment in your stomach in ways that affect how other drugs are absorbed into your bloodstream.

There are two main interaction mechanisms:

• pH change: Raising stomach pH can increase or decrease the absorption of pH-sensitive drugs (most interactions involve reduced absorption).
• Chelation: Aluminum and magnesium ions bind directly to certain drug molecules, forming insoluble complexes that your body can't absorb. This is the primary mechanism for antibiotic interactions.

Major Drug Interactions to Know

These are the most clinically significant interactions. Avoid taking these medications within 2 hours of an aluminum/magnesium antacid (or 4 hours for levothyroxine):

1. Fluoroquinolone Antibiotics

Examples: ciprofloxacin (Cipro), levofloxacin (Levaquin), moxifloxacin (Avelox)

Aluminum and magnesium ions chelate fluoroquinolone antibiotics, dramatically reducing their absorption by up to 90%. This can mean the antibiotic isn't working effectively against your infection. Always take fluoroquinolones at least 2 hours before or 6 hours after taking an antacid.

2. Tetracycline Antibiotics

Examples: doxycycline, minocycline, tetracycline

Antacids substantially reduce tetracycline absorption through chelation. If you're taking doxycycline for a skin condition, Lyme disease, or other infection, take it at least 2 hours before or 2 hours after any antacid dose.

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3. Levothyroxine (Thyroid Medication)

Examples: Synthroid, Levoxyl, Tirosint, generic levothyroxine

This is critical for thyroid patients. Antacids containing aluminum or magnesium can reduce levothyroxine absorption and prevent your thyroid medication from working properly. This interaction is serious enough that thyroid medications should be taken at least 4 hours away from any antacid — ideally on an empty stomach, 30-60 minutes before breakfast.

4. Iron Supplements (Ferrous Sulfate)

Antacids raise stomach pH, which reduces iron absorption. If you take iron supplements for anemia or iron deficiency, take them at least 2 hours away from any antacid. Iron is best absorbed in an acidic environment, so antacids directly counteract iron absorption.

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5. Antifungal Medications (Azole Antifungals)

Examples: ketoconazole, itraconazole, posaconazole

Many azole antifungals require an acidic stomach environment for absorption. Antacids that raise gastric pH can significantly reduce the effectiveness of these antifungals. If you're taking one of these medications, ask your doctor or pharmacist about timing and whether an antacid is appropriate.

6. Aspirin and Salicylates

Antacids can increase the urinary elimination of aspirin, reducing its effectiveness. This is less clinically significant for low-dose aspirin (81mg for cardiac protection) but should be noted if you're taking higher-dose aspirin for pain or inflammation.

The 2-Hour Rule (and the 4-Hour Exception)

The general recommendation from pharmacists and prescribers: take aluminum hydroxide/magnesium hydroxide at least 2 hours before or 2 hours after any other oral medication. This gives most medications enough time to be absorbed before the antacid changes your stomach environment.

Exception: levothyroxine should be separated by at least 4 hours. Some antiretrovirals and antifungals may need even longer separation or should be taken with specific beverages to optimize absorption — check with your pharmacist.

What to Tell Your Doctor or Pharmacist

Always tell your healthcare provider that you're taking aluminum hydroxide/magnesium hydroxide antacids if you're:

• Starting a new prescription medication (ask your pharmacist to check for interactions)
• Being treated for an infection with antibiotics
• Taking thyroid medication, iron supplements, or antifungal drugs
• On antiretroviral therapy for HIV

For information on side effects, see our guide on aluminum hydroxide/magnesium hydroxide side effects. If you're looking for this medication at a pharmacy near you, medfinder can call pharmacies on your behalf and text you which ones have it in stock.