Chemistry of antacids

Chia Jia Yan
Lee Dang Ni
Lim Chai Ying
Lim Ren Hann
Types of antacids
Usage and treatment
Mode of action & reaction
Side effects
Combination Drugs
Stomach contains gastric acid of pH of 2 to 3.
Gastric acid contains HCl, KCl and NaCl.
Excess acid can cause pH to fall below 2 which
can cause problems such as abdominal pain and
heartburn etc.
Gastric juice activates pepsin, an enzyme that
carries out proteolysis – break down proteins by
breaking bonds that links amino acids
Parietal cell produce gastric acid using
proton pump H+/K+ ATPase, an
Epithelial cells
As a proton pump, it transport 1 H+ in
exchange of 1 K+ from stomach against
concentration gradient with ATP
providing the energy.
Picture taken from My Optum Health.
+hea?section=2 (accessed on 8 April 2009)
Alkaline salt or buffer substances
used to neutralize stomach acid
and bring its pH back to 2 to 3
Treat indigestion or relieve any
discomfort caused by acidity of
stomach acid
Reduces acid concentration
within the lumen of the
esophagus which increase the
intra-esophageal pH and decrease
pepsin activity
In forms of tablet, liquid suspension , lonzenges, chewing
gum, dissolving tablet
Liquid relief symptoms faster
Active ingredient: Basic metal salt
Cations used are highlighted in Red
Anions used: OH-, O2-, CO32-, HCO3-, HPO3-, Trisilicate (Mg), amino
acetate (Al)
Commonly used: Al(OH)3, MgOH, CaCO3
By mixing and matching cations and anions, combining
different types of antacids, unique attributes, properties and
potency of antacids are created.
Either Mixture or Complex antacids
Other common ingredient:
Simethicone – relieve gas by breaking down bubbles
Alginic acid – foaming agent that floats on top of
stomach content
Calcium Carbonate
 Most potent antacid
ingredient; acts rapidly
with more prolonged
action than sodium
 Fast acting and long
lasting effect
 Good when patient
suffers from calcium
Magnesium Salts
 Can use hydroxide,
phosphate and trisilicate
(common in Singapore)
 Less potent that Ca
 hydroxide has the
highest potency
 Slow acting
 Magnesium antacids are
generally NOT
absorbed. Any small
amounts are cleared
Aluminium Salts
(usually hydroxide)
 Most stable form of
aluminium salts under
normal conditions
 May be dehydrated to
form powder that readily
dissolves in acids
 Mild and slow acting
antacid, last longer
 Insoluble in water and
forms a suspension/gel
that coats and protects the
stomach lining
 Most appropriate if
patient suffers from renal
Calcium Carbonate
 Alka-mints tablets
 Childrens’ Mylanta
 Chooz Gum
 Alcalak
 Titralac
Magnesium Salts
 Milk of Magnesia
 Philips Tablets
 Philips Oral
Aluminium Salts
(usually hydroxide)
 Maalox
 Mylanta
 ALternaGEL
Antacids needs to be in hydrated form for better
neutralizing effect
Aluminium Antacids
 Aluminium based antacids preferred for kidney failure
patient as it can be used to remove excess phosphate ions
Other usage:
Magnesium used as a laxative for constipated patient
Strength of an antacid to neutralize acid in the stomach is determined
using the antacid’s neutralizing capacity (ANC)
ANC is expressed as milliequivalents (mEq) of the amount of 1N HCl
that can be neutralized
FDA: all antacids must have a neutralizing capacity of at least 5 mEq
per dose.
The commonly used antacids are ranked in this order with respect to
ANC, from strongest to weakest
CaCO3 > NaHCO3 > Mg(OH)2 > Al(OH)3
CaCO3 + 2 HCl  CaCl2 + H2O + CO2
1g will neutralize 20mEq of acid
CaCl2 + CO32-  CaCO3 + Cl- (higher pH in intestine)
Some unchange calcium is absorbed by the gut, which
can raise the pH of the blood causing alkalosis – can
affect proteins
Magnesium oxides, hydroxides and carbonates are poorly
soluble, only Chloride are soluble.
Mg(OH)2 + 2HCl  MgCl2 + 2H2O
1 g can neutralize 2.7 mEq of acid
MgCl + HCO3-  MgCO3 +
Although non-absorbable, 5% - 10% of Mg enter systemic
circulation which then rapidly removed by kidney
Al(OH)3 + 3HCl  AlCl3 + 3H2O Al(H2O)63+
1 g can neutralize 0.4 – 1.8 mEq of acid
Solubility of Al increases as pH decrease
Complex antacids can be used.
E.g. Magaldrate [Mg(OH)Al2(OH)10]4-.2H2O
Causes constipation
Relaxation of the gastrointestinal smooth muscle
delay in stomach emptying
Form insoluble complex of aluminum phosphate (AlPO4),
which is excreted in the feces. May lead to lowered serum
phosphate concentrations and phosphorus mobilization
from the bone. If phosphate depletion is already present,
osteomalacia, osteoporosis, and fracture may result
BUT it reduce phosphates in the urine and prevent
formation of phosphatic (struvite) urinary stones
Causes diarrhea:
1. Mg2+ draw water from the surrounding body tissues into
the intestinal tract by osmosis.
2. Higher quantity of water in the intestinal tract softens and
increases the volume of feces, stimulating nerves in the
3. Mg2+ also play a role in releasing the peptide hormone
cholecystokinin, causing accumulation of water and
electrolytes in the intestine and triggering intestinal
Magnesium salts may cause central nervous depression in
the presence of renal insufficiency
Causes hypermagnesaemia in patients with severe renal
function impairment
BUT Magnesium hydroxide inhibits the precipitation of
calcium oxalate and calcium phosphate, thus preventing the
formation of calcium stones
Release of CO2 cause belching, nausea, abdominal
distention, and flatulence.
Calcium may induce rebound acid secretion.
Calcium stone (kidney stone) can be formed.
Excess Ca2+ cause hypercalcemia. Not a problem in normal
patients. But 3 - 4 g of CaCO3 per day can be problematic in
patients with uremia.
Antacids may affect drugs by altering gastric and urinary
pH, (e.g., thyroid hormones)
Al3+ and Mg2+ antacids are notable for their propensity to
chelate other drugs present in the GI tract, forming
insoluble complexes that pass through the GI tract without
Most interactions can be avoided by taking antacids 2 hours
before or after ingestion of other drugs
Require large neutralizing capacity
single dose (156 meq) antacid 1 hr after meal  neutralize
gastric acid for 2 hr
2nd dose 3 hr after eating  maintains effect for > 4 hr
Tablet antacids generally weaker  large number
Very inconvenient to adminster
NiDDK Image Library,

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