Overview
Provide immediate relief from heartburn — work in minutes but only last 1-2 hours. Best for occasional symptoms or breakthrough on top of regular PPI/H2 treatment.
What it is used for
Occasional heartburn (less than twice a week)
Breakthrough symptoms while on PPI or H2 blocker
Nighttime reflux (especially Gaviscon Advance)
Pregnancy heartburn (preferred over PPIs in mild cases)
Common side effects
Aluminum-containing antacids: constipation
Magnesium-containing antacids: diarrhea
Calcium-containing antacids: constipation, kidney stones if excessive
Most products combine these to balance the side effects
When to See a Doctor
Do not stop a prescribed medicine on your own. Speak to your doctor or pharmacist about side effects, interactions or any doubt, and seek urgent care for the serious warning signs above.
Drug class and examples
Antacids (acid neutralizers) and alginates
Maalox, Mylanta (aluminum + magnesium hydroxide), Tums (calcium carbonate), Gaviscon and Gaviscon Advance (alginate that forms a raft on top of stomach contents — very useful for nighttime reflux), Rennie (calcium + magnesium carbonate).
How it works
Antacids neutralize stomach acid that is already there — providing fast relief but no protection against new acid being produced. Alginates (in Gaviscon) form a foam "raft" that floats on top of stomach contents and physically blocks reflux for 30-60 minutes.
How to take it
Take when symptoms occur. Liquid forms work faster than tablets. Chew tablets thoroughly. For maximum effect: take 1 hour after meals and at bedtime. Don't take within 2 hours of other medications (interferes with absorption).
Serious side effects — act on these
• Severe diarrhea or weakness with chronic high-dose use — Possible electrolyte disturbance — see your doctor.
• Black stools NOT explained by other causes (some bismuth-containing antacids cause this) — Investigate to rule out bleeding.
Interactions
Antacids reduce absorption of MANY medications: thyroid hormones, antibiotics (tetracycline, fluoroquinolones), iron, calcium, certain blood thinners. Always separate from other medications by 2-4 hours.
Monitoring
No routine monitoring. Long-term high-dose use of calcium antacids: check serum calcium and kidney function. Avoid in advanced kidney disease (electrolyte issues).
Pregnancy and breastfeeding
Most antacids are safe in pregnancy. Calcium-based (Tums, Rennie) are often preferred. AVOID those containing aspirin (Alka-Seltzer) and bismuth (Pepto-Bismol) in pregnancy.
Questions to ask your doctor
• Should I take antacid before or after meals?
• Is Gaviscon Advance better than regular Gaviscon?
• Can I take antacids during pregnancy?
Source and review
This guide comes from the Saudi Gastroenterology Association (SGA) patient library, prepared by the SGA Patient Education Panel. Last reviewed: 2026-04-24. It is for education and does not replace your doctor's advice.
Frequently Asked Questions
What is Antacids (Maalox, Gaviscon, Rennie) used for?
How does Antacids (Maalox, Gaviscon, Rennie) work?
What are the side effects of Antacids (Maalox, Gaviscon, Rennie)?
When should I call my doctor about Antacids (Maalox, Gaviscon, Rennie)?
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