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Antacids are medicines that neutralize acid in the stomach. They contain ingredients such as aluminum, calcium, magnesium, or sodium bicarbonate which act as bases (alkalis) to counteract stomach acid and make its pH more neutral.

pH is a measure of the concentration of hydrogen ions in a solution and this determines how acidic or how alkaline that solution is. The scale ranges from 1 to 14, where below 7 is acidic, 7 is neutral, and above 7 is alkaline. Normal gastric acid pH is in the range 1.5-3.5.

What are antacids used for?

Antacids are used to relieve the symptoms of gastroesophageal reflux disease (GERD, also known as acid reflux), heartburn or indigestion (also called dyspepsia). By neutralizing stomach acid, antacids relieve symptoms such as burning behind the breast bone or throat area (esophagus) caused by acid reflux, a bitter taste in the mouth, a persistent dry cough, pain when lying down, or regurgitation.

Some antacids are also used for completely unrelated medical conditions, for example:

• Aluminum antacids: lower elevated blood phosphate levels and prevent the formation of kidney stones
• Calcium carbonate antacids: treat calcium deficiency
• Magnesium oxide antacids: treat magnesium deficiency.

What are the differences between antacids?

The two main differences between antacids is the ingredients they contain and their formulation. The different ingredients - aluminum, calcium, magnesium, or sodium bicarbonate – all have differences in how long they take to start working, how long they keep working for, what other medications they may interact with, and who they are suitable for.

Antacids are available as liquids or tablets. Some products combine several antacid ingredients together or include alginates. Alginates are gum-like substances that float on top of the stomach contents, forming a raft that acts like a barrier. These may provide more symptom relief in people with reflux.

Some products also contain other ingredients that are not antacids or alginates, such as simethicone which helps disperse gas in people prone to bloating. Aspirin also features in some products (for example Alka-Seltzer).

Are antacids safe?

When taken at the recommended dosage, for short durations of time, by people without contraindications, antacids are considered safe.

However, antacids only relieve symptoms and will not cure the underlying problem causing the symptoms. This means they may mask some serious conditions, such as gastric cancer. If you are taking antacids more than two to three times a week for longer than two weeks you should talk to your doctor. Even if your condition is diagnosed as GERD, other medications (such as H2 antagonists, proton pump inhibitors), that work by reducing or preventing the secretion of stomach acid rather than just neutralizing it, may be more appropriate.

Some antacids contain sodium bicarbonate which can affect blood pressure and circulation, particularly those with pre-existing problems. High intakes of sodium are also not recommended for people with liver or kidney disease. Anybody on a sodium-controlled or low-sodium diet or prone to high blood pressure, kidney or liver disease should talk with their doctor before taking antacids containing sodium bicarbonate.

Always speak to your doctor if you are having reflux-like symptoms which do not go away with antacid use. Some symptoms of heart disease (such as angina or a heart attack) are like those of reflux or heart burn.

Interactions may occur with some ingredients contained in antacid products if they are used with other medications. For example, calcium can affect the absorption of medicines such as bisphosphonates, tetracyclines, and ferrous sulfate (iron). Antacids may also alter the absorption or excretion of some other medicines (such as digoxin, phenytoin, and pseudoephedrine).

In 2016, the FDA issued a safety alert about antacids that contain aspirin, such as Alka-Seltzer because their use was associated with reports of serious bleeding, particularly in people also taking other medications that increased the risk of bleeding.

Antacids should not be given to children aged less than six years. They may not be safe in people with certain medical conditions, such as gastrointestinal bleeding, liver or kidney disease, or heart disease. Women who are pregnant or breastfeeding should also talk to their pharmacist or doctor before using antacids.

What are the side effects of antacids?

The side effects of antacids directly relate to the ingredients they contain. Some combination products have side effects that cancel each other out (for example, aluminium causes constipation and magnesium causes diarrhea so products containing a combination of these two ingredients are more likely to have a neutral effect on the bowel).

Most products have few side effects when taken as directed on the label. Side effects are more prevalent when the product is overused or taken for longer than intended. Side effects associated with common antacid ingredients include (some are rare):

• Aluminum: constipation, low blood phosphate levels, aluminium toxicity, osteomalacia
• Calcium: nausea, vomiting, kidney stones, high blood calcium levels, alkalosis
• Magnesium: diarrhea, nausea, vomiting, high blood magnesium levels
• Sodium bicarbonate: increased blood pressure, nausea, bloating, gas.

Antacids that are consumed at too high a dose for too long a period can also cause a condition called acid rebound. This is when the stomach produces even more acid after food and drinks have been consumed.