Antacids have been used in the treatment of dyspepsia for at least 2,000 years (Walan, 1984). The main aim of treatment with antacid is to reduce the acidity of gastric contents. This has an additional effect of decreasing peptic activity if the pH becomes higher than 3.5 to 4 since at this level the activity of pepsin starts to fall markedly. There may be other benefits such as the bile acid binding capacity of some of the aluminium-containing antacids. This is useful in conditions where bile reflux is playing an aetiological role in the development of gastric ulcer. The ideal antacid should have a rapic reaction with acid, a high acid-binding capacity and buffering effect in the pH range 3-5.It should have little effect on gastrointestinal mobility and must not be absorbed enough to cause systemic side-effects. Antacids are usually called in scientific name : Sodium bicarbonate, Magnesium hydroxide and Aluminium hydroxide.
Sodium bicarbonate has a long history of use but has become less popular because of its tendency to cause systemic effects. It is absorbed from the gastrointestinal tract and a slight alkalosis develops, with the production of alkaline urine. Calcium carbonate is also a potent antacid. However, with this drug, systemic absorption of calcium can occur. Hypercalcaemia plus the systemic alkalosis which it also causes sometimes results in the milk-alkali syndrome. Like most other antacids it increases the gastric acid output when ingested a meal. With calcium carbonate, however, this effect seems to be due to a specific stimulatory action of calcium on the parietal cell and is not just a consequence of its buffering action. As a result of the potential for serious side-effects, both sodium bicarbonate and calcium carbonate have been used less and less in recent years.
Magnesium hydroxide is constituent of most modern antacids. It is favoured because its administration is not likely to result in severe systemic effects. Significant absorption does not usually take place due to it being highly insoluble in water. In patients with impaired renal function, about 5% to 10%of the initial oral load can be absorbed, giving rise to systemic problems. This tendency would glomerular filtration rates are common.
Aluminium hydroxide, like magnesium hydroxide, is usually classified as non-systemic antacid. It reacts in the small bowel with phosphate to form insoluble salts and is thus absorbed only to a very limited extent. However, in one study of patients with normal renal function, using standard dosage, the aluminium concentration in the serum was approximately doubled. Furthermore, there was a fivefold increase in urinary aluminium excretion. Whether this aluminium absorption is of any clinical significance is uncertain.
Aluminium hydroxide can cause the severe clinical condition of hypophophataemia by its binding action on phosphate in the small bowel. It also causes a decrease in the absorption of fluoride. If this decrease in fluoride absorption were to result insignificant demineralization of bone, it wound contraindicate its use in the elderly who are prone to osteoporosis and osteomalacia. Like other antacids, aluminium hydroxide can decrease the availability of other drugs. Of particular importance are the reports of diminished absorption of cimetidine by antacids and the decreased bioavailability of prednisolone in patients on antacids.
Suggestions that aluminium hydroxide has an additional therapeutic effect by local stimulation of prostaglandin secretion require substantiation.
Magaldrate and almasilate are antacids that have become available within the last few years. They are novel in the sense that they are discrete chemical entities with complex crystalline structures. they thus differ from the conventional antacids described above. However, despite claims of great efficacy in the relief of dyspepsia and the healing of gastric ulcer, there is no firm evidence that they differ clinically from the other antacids.(2)
Dosage and Administration
Antacids are administered orally. The dose of antacids should be expressed in trems of mEq of acid neutralizing capacity. Dose and frequency of administration depend on the acid secretory rate of the stomach, gastric emptying time and the disorder being treated. The duration of action of antacids is determined principally of action of 20-60 minutes. However, if the drug are administered 1 hour after meal, acid neutralizing effects may persist up to 3 hours. Sodium bicarbonate generally has a shorter duration of action than other antacids. Antacids should be used for longer than 2-week period only under the management of a physician and as part of a carefully planned therapeutic regimen.
There is considerable variation in in vivo acid neutralizing capacity of equal volumes of different antacids and antacids products. Since suspensions are more rapidly and effectively solubilized than powders or tablets, antacid suspensions have a greater ability to react with and neutralize gastric acid. Antacid suspension have a smaller particle size than do tablets and drying of antacid suspensions to prepare powders and tablets causes substantial loss of ability to neutralize acid. In general, an antacid suspension is preferable to a tablet or powder. Tablets should be reserved for chronic use in patients who refuse suspensions because they are inconvenient or unpalatable. Tablets should be thoroughly chewed before swallowing.
The US Food and Drug Administration (FDA) requires that antacids have a minimum in vitro acid neutralizing capacity of 5 mEq per dose and that antacid labeling contain the in vitro acid neutralizing capacity. However, this FDA in vitro test does not correlate with in vivo acid neutralizing capacity.
For peptic ulcer disease, dosages of antacids are empirical and various antacid dosages have been used. In patients with uncomplicated gastric ulcers, an antacid is administered 1 and 3 hours postprandially and at bedtime. Antacids are administered until healing is complete. Additional doses of antacids may administered to relieve ulcer pain which occurs between regularly scheduled uses. (3)
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