C H A P T E R 5 7
Drugs affecting gastrointestinal secretions
905
hyperacidity and hiatus hernia. See Table 57.1 for usual
indications for each antacid.
Pharmacokinetics
Sodium bicarbonate, the oldest drug in this group, is
readily available in many preparations, including baking
soda powder, tablets, solutions and as an injectable for
treating systemic acidosis. This drug is widely distrib-
uted when absorbed orally, reaching peak levels in 1 to
3 hours, crossing the placenta and entering breast milk.
It is excreted in urine and can cause serious electrolyte
imbalance in people with renal impairment.
Calcium carbonate is actually precipitated chalk and
is available in tablet and powder forms. The main draw-
backs to this agent are constipation and acid rebound.
It has an onset of action in about 3 to 5 minutes. It
can be absorbed systemically and cause calcium imbal-
ance. When absorbed, it is metabolised in the liver
and excreted in urine and faeces, with a half-life of
1 to 3 hours. Calcium carbonate is known to cross the
placenta and enter breast milk.
Magnesium salts are very effective in buffering acid
in the stomach but have been known to cause diarrhoea;
they are sometimes used as laxatives. They are available
as tablets, chewable tablets and capsules and in liquid
forms. Although these agents are not generally absorbed
systemically and are excreted in the faeces, absorbed
magnesium can lead to nerve damage and even coma, if
absorbed; they are excreted in the urine.
Aluminium salts, available as tablets, capsules, sus-
pensions and in a liquid form, do not cause acid rebound
but are not very effective in neutralising acid. They are
bound in faeces for excretion. They have been related
to severe constipation. Aluminium binds dietary phos-
phates and causes hypophosphataemia, which can then
cause calcium imbalance throughout the system.
Aluminium and magnesium minimise the GI effects
of constipation and diarrhoea by combining these two
salts but may cause a rebound hyperacidity and alkalosis.
Many of these antacids are available in combination
forms to take advantage of the acid-neutralising effect
and block adverse effects. For example, a combination
of calcium and aluminium salts (
Mylanta
) buffers acid
and produces neither constipation nor diarrhoea.
Contraindications and cautions
The antacids are contraindicated in the presence of
any known allergy to antacid products or any compo-
nent of the drug
to prevent hypersensitivity reactions
.
Caution should be used in the following instances: any
condition that can be exacerbated by electrolyte or acid–
base imbalance
to prevent exacerbations and serious
adverse effects
; any electrolyte imbalance,
which could
be exacerbated by the electrolyte-changing effects of
these drugs
; GI obstruction,
which could cause systemic
absorption of the drugs and increased adverse effects
;
renal dysfunction,
which could lead to electrolyte dis-
turbance if any absorbed antacid is not neutralised
properly
; and pregnancy and breastfeeding
because of
the potential for adverse effects on the fetus or neonate.
Adverse effects
The adverse effects associated with these drugs relate
to their effects on acid–base and electrolyte balance.
Administering an antacid frequently causes
acid
rebound
, in which the stomach produces more acid in
response to the alkaline environment. Neutralising the
stomach contents to an alkaline level stimulates gastrin
production to cause an increase in acid production and
return the stomach to its normal acidic state. In many
cases, the acid rebound causes an increase in symptoms,
which results in an increased intake of the antacid.
This leads to more acid production and an ongoing
cycle. When more and more antacid is used, the risk for
systemic effects rises. Alkalosis with resultant metabolic
changes (nausea, vomiting, neuromuscular changes,
headache, irritability, muscle twitching and even coma)
may occur. The use of calcium salts may lead to hyper-
calcaemia and milk-alkali syndrome (seen as alkalosis,
renal calcium deposits or severe electrolyte disorders).
Constipation or diarrhoea may result, depending on
the antacid being used. Hypophosphataemia can occur
with the use of aluminium salts. Finally, fluid retention
and heart failure can occur with sodium bicarbonate
because of its high sodium content.
Drug–drug interactions
Antacids can greatly affect the absorption of drugs
from the GI tract. Most drugs are prepared for an acidic
environment, and an alkaline environment can prevent
them from being broken down for absorption or can
actually neutralise them so that they cannot be absorbed.
People taking antacids should be advised to separate
them from any other medications by 1 to 2 hours.
If the pH of urine is affected by large doses of
antacids, levels of drugs, such as quinidine, may increase,
and levels of salicylates may decrease.
Care considerations for
people receiving antacids
Assessment: History and examination
■
■
Assess for
possible contraindications or cautions
:
any history of allergy to antacids
to prevent
hypersensitivity reactions
; renal dysfunction,
which might interfere with the drug’s excretion
;
electrolyte disturbances,
which could be
exacerbated by the effects of the drug
; and current
