DƯỢC LÍ Goodman & Gilman's The Pharmacological Basis of Therapeutics 12th, 2010

Table 45–2
Composition and Acid Neutralizing Capacities of Popular Antacid Preparations
ACID NEUTRALIZING
PRODUCT
a
Al(OH) 3
a
Mg(OH) 2
a
CaCO 3
SIMETHICONE a CAPACITY b
Tablets
Gelusil 200 200 0 25 10.5
Maalox Quick Dissolve 0 0 600 0 12
Mylanta Double Strength 400 400 0 40 23
Riopan Plus Double Strength Magaldrate, 1080 20 30
Calcium Rich Rolaids 80 412 0 11
Tums EX 0 0 750 0 15
Liquids
Maalox TC 600 300 0 0 28
Milk of Magnesia 0 400 0 0 14
Mylanta Maximum Strength 400 400 0 40 25
Riopan Magaldrate, 540 0 15
a
Contents, milligrams per tablet or per 5 ml.
b
Acid neutralizing capacity, milliequivalents per tablet or per 5 ml.
The U.S. marketplace for antacids is fluid. The current trend of “reusing” well-known brand names to introduce new products that contain an active
ingredient different from expected is a source of confusion that can present a danger to patients. Medication safety experts encourage clinical practitioners
to refer to the active ingredient(s) in conjunction with the proprietary (brand) name when selecting OTC products.
The relative effectiveness of antacid preparations is expressed
as milliequivalents of acid-neutralizing capacity (defined as the
quantity of 1N HCl, expressed in milliequivalents, that can be
brought to pH 3.5 within 15 minutes); according to FDA requirements,
antacids must have a neutralizing capacity of at least 5 mEq
per dose. Due to discrepancies between in vitro and in vivo neutralizing
capacities, antacid doses in practice are titrated simply to
relieve symptoms. For uncomplicated ulcers, antacids are given
orally 1 and 3 hours after meals and at bedtime. This regimen, providing
~120 mEq of a Mg-Al combination per dose, may be almost
as effective as conventional dosing with an H 2
receptor antagonist.
For severe symptoms or uncontrolled reflux, antacids can be given
as often as every 30-60 minutes. In general, antacids should be
administered in suspension form because this probably has a greater
neutralizing capacity than powder or tablet dosage forms. If tablets
are used, they should be thoroughly chewed for maximum effect.
Antacids are cleared from the empty stomach in ~30 minutes.
However, the presence of food is sufficient to elevate gastric pH to
~5 for ~1 hour and to prolong the neutralizing effects of antacids for
~2-3 hours.
Antacids vary in the extent to which they are absorbed, and
hence in their systemic effects. In general, most antacids can elevate
urinary pH by ~1 pH unit. Antacids that contain Al 3+ , Ca 2+ , or Mg 2+
are absorbed less completely than are those that contain NaHCO 3
.
With normal renal function, the modest accumulations of Al 3+ and
Mg 2+ do not pose a problem; with renal insufficiency, however,
absorbed Al 3+ can contribute to osteoporosis, encephalopathy, and
proximal myopathy. About 15% of orally administered Ca 2+ is
absorbed, causing a transient hypercalcemia. Although this is not a
problem in normal patients, the hypercalcemia from as little as 3-4 g
of CaCO 3
per day can be problematic in patients with uremia. In the
past, when large doses of NaHCO 3
and CaCO 3
were administered
commonly with milk or cream for the management of peptic ulcer,
the milk-alkali syndrome (alkalosis, hypercalcemia, and renal insufficiency)
occurred frequently. Today, this syndrome is rare and generally
results from the chronic ingestion of large quantities of Ca 2+
(five to forty 500-mg tablets per day of calcium carbonate) taken
with milk. Patients may be asymptomatic or may present with the
insidious onset of hypercalcemia, reduced secretion of parathyroid
hormone, retention of phosphate, precipitation of Ca 2+ salts in the
kidney, and renal insufficiency.
By altering gastric and urinary pH, antacids may affect a
number of drugs (e.g., thyroid hormones, allopurinol, and imidazole
antifungals, by altering rates of dissolution and absorption, bioavailability,
and renal elimination). Al 3+ and Mg 2+ antacids also are
notable for their propensity to chelate other drugs present in the GI
tract, forming insoluble complexes that pass through the GI tract
without absorption. Thus it generally is prudent to avoid concurrent
administration of antacids and drugs intended for systemic absorption.
Most interactions can be avoided by taking antacids 2 hours
before or after ingestion of other drugs.
Other Acid Suppressants and Cytoprotectants
The M 1
muscarinic receptor antagonists pirenzepine and telenzepine
(Chapter 9) can reduce basal acid production by 40-50% and long
have been used to treat patients with peptic ulcer disease in countries
other than the U.S. The ACh receptor on the parietal cell itself is of
the M 3
subtype, and these drugs are believed to suppress neural stimulation
of acid production via actions on M 1
receptors of intramural
ganglia (Figure 45–1). Because of their relatively poor efficacy, significant
and undesirable anticholinergic side effects, and risk of
blood disorders (pirenzepine), they rarely are used today.