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Antacids either directly neutralize acidity, increasing the pH, or reversibly reduce or block the secretion of acid by gastric cells to reduce acidity in the stomach. When gastric hydrochloric acid reaches the nerves in the gastrointestinal mucosa, they signal pain to the central nervous system. This happens when these nerves are exposed. In addition to the reduction of gastric acidity, antacids also alter the profile of prostaglandins produced by gastroduodenal mucosa and this may promote mucosal healing and be related to its therapeutic effect.
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. The treatment of ulcers resulting from excessive acidity may require H2-receptor antagonists or proton pump inhibitors, and eradication of H. pylori.
Excess calcium from supplements, fortified food and high-calcium diets, can cause milk-alkali syndrome, which has serious toxicity and can be fatal. In 1915, Bertram Sippy introduced the "Sippy regimen" of hourly ingestion of milk and cream, the gradual addition of eggs and cooked cereal, for 10 days, combined with alkaline powders, which provided symptomatic relief for peptic ulcer disease. Over the next several decades, the Sippy regimen resulted in renal failure, alkalosis, and hypercalcemia[clarification needed], mostly in men with peptic ulcer disease. These adverse effects were reversed when the regimen stopped, but it was fatal in some patients with protracted vomiting. Milk alkali syndrome declined in men after effective treatments were developed for peptic ulcer disease. But during the past 15 years, it has been reported in women taking calcium supplements above the recommended range of 1200 to 1500 mg daily, for prevention and treatment of osteoporosis, and is exacerbated by dehydration. Calcium has been added to over-the-counter products, which contributes to inadvertent excessive intake. The New England Journal of Medicine reported a typical case of a woman who arrived in the emergency department vomiting and in altered mental status, writhing in pain. She had consumed large quantities of chewable antacid tablets containing calcium carbonate. She gradually recovered.
Other adverse effects from antacids include:
Side effects from antacids vary depending on the individual, and on other medications they may be taking at the time. Those who experience side effects most commonly suffer from changes in bowel functions, such as diarrhea, constipation, or flatulence.
Although reactions to any drug may vary from person to person, generally those medications that contain aluminum or calcium are the likeliest to cause constipation, those that contain magnesium are the likeliest to cause diarrhea. Some products combine these ingredients, which essentially cancels them out, to forestall unpleasant side effects.
Heartburn, reflux, indigestion, and sour stomach are a few of the common terms used to describe digestive upset. Self-diagnosis of indigestion does carry some risk because the causes can vary from a minor dietary indiscretion to a peptic ulcer. The pain and symptoms of gastro-oesophageal reflux disease, GERD or simply "reflux", may mimic those of a heart attack. Misdiagnosis can be fatal. A bleeding ulcer can be life threatening. GERD, and pre-ulcerative conditions in the stomach are treated much more aggressively since both, if untreated, could lead to oesophageal or stomach cancer. It is primarily for this reason that the histamine H2 antagonists including cimetidine (Tagamet), famotidine (Pepcid), and ranitidine (Zantac), and the proton pump inhibitor (PPI) omeprazole (Prilosec) were made available over the counter. These drugs stop production of stomach acid and provide longer lasting relief but they do not neutralize any stomach acid already present in the stomach.
Antacid tablets dissolved in water is also used as a temporary remedy to sooth pain induced by lachrymatory agents, commonly known as tear gas. They have been used extensively during the recent and ongoing anti-government protests in Turkey.
Altered pH or complex formation may alter the bioavailability of other drugs, such as tetracycline and amphetamine. Urinary excretion of certain drugs may also be affected. Chelation of tetracycline with aluminium hydroxide can cause nausea, vomiting, and phosphate excretion, resulting in phosphate deficiency.
Antacid preparations, by suppressing acid mediated break down of proteins, lead to an elevated risk of developing food and drug allergies. This happens due to undigested proteins passing into the gastrointestinal tract. It is unclear whether this risk occurs only with long-term use or with short-term use as well. 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. It could also result in reduced bioavailability of some drugs. For example, the bioavailability of ketoconazole (antifungal) is reduced at high intragastric pH (low acid content).
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