These are basic substances which neutralize gastric acid and raise pH of gastric contents. Peptic activity is indirectly reduced if the pH rises above 4, because pepsin is secreted as a complex with an inhibitory terminal moiety that dissociates below pH 5.
ANTACIDS
These are basic
substances which neutralize gastric acid and raise pH of gastric contents.
Peptic activity is indirectly reduced if the pH rises above 4, because pepsin
is secreted as a complex with an inhibitory terminal moiety that dissociates
below pH 5: optimum peptic activity is exerted between pH 2 to 4.
Antacids do not
decrease acid production; rather, agents that raise the antral pH to > 4
evoke reflex gastrin release → more acid is secreted, especially in patients
with hyperacidity and duodenal ulcer; “acid rebound” occurs and gastric
motility is increased.
The potency of an
antacid is generally expressed in terms of its acid neutralizing capacity (ANC), which is defined as number of
mEq of 1N HCl that are brought to pH 3.5 in 15 min (or 60 min in some tests) by
a unit dose of the antacid preparation. This takes into consideration the rate
at which it dissolves and reacts with HCl. This is important because a single
dose of any antacid taken in empty stomach acts for 30– 60 min only, since in
this time any gastric content is passed into duodenum. Taken with meals
antacids may act for at the most 2–3 hr.
Systemic Antacids
Sodium Bicarbonate
It is water soluble,
acts instantaneously, but the duration of action is short. It is a potent
neutralizer (1 g → 12 mEq HCl), pH may rise above 7.
However, it has several
demerits:
·
Absorbed systemically: large doses will induce
alkalosis.
·
Produces CO2 in stomach → distention,
discomfort, belching, risk of ulcer perforation.
·
Acid rebound occurs, but is usually short
lasting.
·
Increases Na+ load: may worsen edema and CHF. Use
of sod. bicarbonate is restricted to casual treatment of heartburn: provides
quick symptomatic relief.
Other uses are to
alkalinize urine and to treat acidosis.
Sodium citrate Properties similar to
sod. bicarbonate; 1 g neutralizes 10 mEq
HCl; CO2 is not evolved.
Nonsystemic Antacids
These are insoluble
and poorly absorbed basic compounds; react in stomach to form the corresponding
chloride salt. The chloride salt again reacts with the intestinal bicarbonate
so that HCO3¯ is not spared for absorption—no acid-base disturbance
occurs. However, small amounts that are absorbed have the same alkalinizing
effect as NaHCO3.
Mag. hydroxide has low water
solubility: its aqueous suspension (milk of
magnesia) has low concentration of OH¯ ions and thus low alkalinity. However,
it reacts with HCl promptly and is an efficacious antacid (1 g → 30 mEq HCl). Rebound
acidity is mild and brief.
MILK OF MAGNESIA 0.4
g/5 ml suspension: 5 ml neutralizes 12 mEq acid.
Magnesium trisilicate has low solubility and
reactivity; 1 g can react with 10
mEq acid, but in clinical use only about 1 mEq is neutralized.
About 5% of
administered Mg is absorbed systemically—may cause problem if renal function is
inadequate.
All Mg salts have a laxative action—by generating osmotically
active MgCl2 in the stomach and through Mg2+ ion induced
cholecystokinin release. Soluble Mg salts are used as osmotic purgatives.
Aluminium Hydroxide Gel
It is a bland, weak
and slowly reacting antacid. On
keeping it slowly polymerizes to variable extents into still less reactive
forms. Thus, the ANC of a preparation gradually declines on storage. Also, the
product from different manufacturers may have differing ANCs; usually it varies
from 1–2.5 mEq/g. Thus, 5 ml of its suspension may neutralize just 1 mEq HCl.
As such, little worthwhile acid neutralization is obtained at conventional
doses.
The Al3+ ions relax smooth muscle. Thus, it delays gastric
emptying. Alum. hydrox. frequently, causes constipation due to its smooth
muscle relaxant and mucosal astringent action.
Alum. hydrox. binds phosphate in the intestine and prevents its absorption—hypophosphatemia
occurs on regular use. This may:
·
cause
osteomalacia
·
be used therapeutically in hyperphosphatemia
and phosphate stones.
Small amount of Al3+
that is absorbed is excreted by kidney which is not possible in renal
failure—aluminium toxicity (encephalopathy, osteoporosis) can occur.
ALUDROX 0.84 g tab,
0.6 g/10 ml susp.
Magaldrate
It is a hydrated complex of hydroxymagnesium aluminate that
initially reacts rapidly with acid and releases alum. hydrox. which then reacts
more slowly. The freshly released alum. hydrox. is in the unpolymerized more
reactive form. Thus, magaldrate cannot be equated to a physical mixture of mag.
and alum. hydroxides. It is a good antacid with prompt and sustained
neutralizing action. Its ANC is estimated to be 28 mEq HCl/g.
STACID 400 mg tab, 400 mg/5 ml susp.; ULGEL 400 mg with 20 mg
simethicone per tab or 5 ml susp.
Calcium Carbonate
It is a potent and
rapidly acting acid neutralizer (1 g → 20 mEq HCl), but ANC
of commercial preparations is less and variable due to differing particle size
and crystal structure. Though it liberates CO2 in the stomach at a
slower rate than NaHCO3, it can cause distention and discomfort. The
Ca2+ ions are partly absorbed.
The greatest drawback
of CaCO3 as an antacid is that Ca2+ ions diffuse into the gastric
mucosa—increase HCl production directly by parietal cells as well as by
releasing gastrin. Acid rebound is marked. Cal. carbonate is constipating in most
individuals, but in some it causes loose motions. The absorbed calcium can be
dangerous in renal insufficiency.
Milk Alkali Syndrome
In the past, large
quantity of milk was prescribed with
CaCO3 (or NaHCO3) for peptic ulcer. Such regimen often
produced a syndrome characterized by headache, anorexia, weakness, abdominal
discomfort, abnormal Ca deposits and renal stones due to concurrent
hypercalcaemia and alkalosis. It is rare now.
Antacid Combinations
A combination of two or more antacids is frequently used. These may
be superior to any single agent on the following accounts:
·
Fast (Mag. hydrox.) and slow (Alum. hydrox.)
acting components yield prompt as well as sustained effect.
·
Mag. salts are laxative, while alum. salts are
constipating: combination may annul each other’s action and bowel movement may
be least affected.
·
Gastric emptying is least affected; while
alum. salts tend to delay it, mag./cal. salts tend to hasten it.
·
Dose of individual components is reduced;
systemic toxicity (dependent on fractional absorption) is minimized.
Some available antacid
combinations are:
ACIDIN: Mag. carb. 165
mg, dried alum. hydrox. gel 232 mg, cal. carb. 165 mg, sod. bicarb. 82 mg, with
kaolin 105 mg and belladonna herb 30 μg per tab.
ALMACARB: Dried alum.
hydrox. gel 325 mg, mag. carb. 50 mg, methyl polysilox. 40 mg,
deglycyrrhizinated liquorice 380 mg per tab.
ALLUJELDF: Dried alum.
hydrox. gel 400 mg, mag.
hydrox. 400 mg, methyl
polysilox. 30 mg per 10 ml susp.
DIGENE: Dried alum.
hydrox. gel 300 mg, mag. alum. silicate 50 mg, mag. hydrox. 25 mg,
methylpolysilox. 10 mg per tab.
DIGENE GEL: Mag.
hydrox. 185 mg, alum. hydrox. gel 830 mg, sod. carboxymethyl cellulose 100 mg,
methylpolysilox. 25 mg per 10 ml susp.
GELUSIL: Dried alum. hydrox. Gel 250 mg, mag. trisilicate 500 mg per
tab.
GELUSIL LIQUID: Mag. Trisilicate
625 mg, alum. hydrox. gel 312 mg per 5 ml susp.
MUCAINE: Alum. hydrox.
290 mg, mag. hydrox. 98 mg, oxethazaine 10 mg per 5 ml susp.
TRICAINEMPS: Alum. hydrox.
gel 300 mg, mag. hydrox. 150 mg, oxethazaine 10 mg, simethicone 10 mg per 5 ml
gel.
MAYLOX: Dried alum.
hydrox. gel 225 mg, mag. hydrox. 200 mg, dimethicone 50 mg per tab and 5 ml
susp.
POLYCROL FORTE GEL:
Mag. hydrox. 100 mg, dried alum. hydrox. gel 425 mg, methylpolysilox. 125 mg
per 5 ml susp.
Drug Interactions
By raising gastric pH
and by forming complexes,
the nonabsorbable antacids decrease the absorption of many drugs, especially
tetracyclines, iron salts, fluoroquinolones, ketoconazole, H2
blockers, diazepam, phenothiazines, indomethacin, phenytoin, isoniazid,
ethambutol and nitrofurantoin. Stagger their administration by 2 hours. The
efficacy of nitrofurantoin is also reduced by alkalinization of urine.
Uses
Antacids are no longer
used for healing peptic ulcer because they are needed in large and frequent
doses, are inconvenient, can cause acid rebound and bowel upset, afford little
nocturnal protection and have poor patient acceptability. Antacids are now
employed only for intercurrent pain relief and acidity, mostly selfprescribed
by the patients as over the counter preparations. They continue to be used for
nonulcer dyspepsia and minor episodes of heartburn.
Gastroesophageal
reflux Antacids afford faster symptom relief than drugs which inhibit acid
secretion, but do not provide sustained benefit. May be used off and on for
acid eructation and heartburn.
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