Antacids

 

any one of several basic substances that counteract stomach acidity (see stomach). Antacids are used by physicians to treat hyperchlorhydria, i.e., the excessive production of hydrochloric acid by the parietal cells lining the stomach. Commonly used antacid preparations, most sold without prescription, contain sodium bicarbonate, magnesium hydroxide, or aluminum hydroxide. Some preparations contain substances such as magnesium trisilicate that reduce the formation of gas.

An antacid is any substance, generally a base, which counteracts stomach acidity. In other words, antacids are stomach acid neutralizers.

Action mechanism

Antacids perform a neutralization reaction, i.e. they buffer gastric acid, raising the pH to reduce acidity in the stomach. When gastric hydrochloric acid reaches the nerves in the gasitrointestinal mucosa, they signal pain to the central nervous system. This happens when these nerves are exposed, as in peptic ulcers. The gastric acid may also reach ulcers in the esophagus or the duodenum.

Other mechanisms may contribute, such as the effect of aluminum ions inhibiting smooth muscle cell contraction and delaying gastric emptying.

Indications

Antacids are taken by mouth to relieve heartburn, the major symptom of gastroesophageal reflux disease, or acid indigestion. Treatment with antacids alone is symptomatic and only justified for minor symptoms. Peptic ulcers may require H2-receptor antagonists or proton pump inhibitors.

The usefulness of many combinations of antacids is not clear, although the combination of magnesium and aluminum salts may prevent alteration of bowel habits.

Side effects

• Aluminum hydroxide: may lead to the formation of insoluble aluminum-phosphate-complexes, with a risk for hypophosphatemia and osteomalacia. Although aluminum has a low gastrointestinal absorption, accumulation may occur in the presence of renal insufficiency. Aluminum-containing drugs may cause obstipation.
• Magnesium hydroxide: has laxative properties. Magnesium may accumulate in patients with renal failure leading to hypermagnesemia, with cardiovascular and neurological complications. See Milk of magnesia.
• Carbonate: regular high doses may cause alkalosis, which in turn may result in altered excretion of other drugs, and kidney stones. A chemical reaction between the carbonate and hydrochloric acid may produce carbon dioxide gas. This causes gastric distension which may not be well tolerated.
• Calcium: compounds containing calcium may increase calcium output in the urine, which might be associated to renal stones. Calcium salts may cause Constipation.
• Sodium: increased intake of sodium may be deleterious for arterial hypertension, heart failure and many renal diseases.

Interactions

Altered pH or complex formation may alter the bioavailability of other drugs, such as tetracycline. Urinary excretion of certain drugs may also be affected.

Problems with reduced stomach acidity

Reduced stomach acidity may result in an impaired ability to digest and absorb certain nutrients, such as iron and the B vitamins. Since the low pH of the stomach normally kills ingested bacteria, antacids increase the vulnerability to infection.

Drug names

Examples of antacids (brand names may vary in different countries).

• Aluminum hydroxide (Amphojel^®, AlternaGEL^®)
• Magnesium hydroxide (Phillips’^® Milk of Magnesia)
• Aluminum hydroxide and magnesium hydroxide (Maalox^®, Mylanta^®)
• Aluminum carbonate gel (Basaljel^®)
• Calcium carbonate (Alcalak^®, Calcium Rich TUMS^®, Quick-Eze^®, Rennie^®, Titralac^®, Rolaids^®)
• Sodium bicarbonate (Bicarbonate of soda, Alka-Seltzer^®)
• Hydrotalcite (Mg₆Al₂(CO₃)(OH)₁₆ · 4(H₂O); Talcid^®)
• Bismuth subsalicylate (Pepto-Bismol)
• Magaldrate + Simethicone (Pepsil)

Major Drug Groups
Gastrointestinal tract (A)	Antacids • Antiemetics  • H₂-receptor antagonists • Proton pump inhibitors • Laxatives • Antidiarrhoeals
Blood and blood forming organs (B)	Anticoagulants • Antiplatelets • Thrombolytics
Cardiovascular system (C)	Antiarrhythmics • Antihypertensives • Diuretics • Vasodilators • Antianginals • Beta blockers • Angiotensin converting enzyme inhibitors • Antihyperlipidemics
Skin (D)	Antipruritics
Reproductive system (G)	Hormonal contraception • Fertility agents • Selective estrogen receptor modulators • Sex hormones
Endocrine system (H)	Anti-diabetics • Corticosteroids • Sex hormones • Thyroid hormones
Infections and Infestations (J, P)	Antibiotics • Antivirals • Vaccines • Antifungals • Antiprotozoals • Anthelmintics
Malignant and Immune disease (L)	Anticancer agents • Immunosuppressants
Muscles, Bones, and Joints (M)	Anabolic steroids • Anti-inflammatories • Antirheumatics • Corticosteroids • Muscle relaxants
Brain and Nervous system (N)	Anesthetics • Analgesics • Anticonvulsants • Mood stabilizers  • Anxiolytics • Antipsychotics • Antidepressants • Nervous system stimulants
Respiratory system (R)	Bronchodilators • Decongestants  • Antihistamines

Drugs for acid related disorders: Antacids (A02A)
Magnesium	Magnesium carbonate - Magnesium oxide - Magnesium peroxide - Magnesium hydroxide - Magnesium silicate
Aluminium	Aluminium hydroxide - Algeldrate - Aluminium phosphate - Dihydroxialumini sodium carbonate - Aluminium acetoacetate - Aloglutamol - Aluminium glycinate
Calcium	Calcium carbonate - Calcium silicate

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